I would argue that the modern theories are much more nuanced and united, and that there is not much controversy left in the question of what caused World War 1: a little bit of everything. You'll find imperialism, nationalism, cultural and diplomatic militarism, the naval race, the alliance systems and the crises in Morocco and the Balkans in most books that pertain to these topics, and you'll find few mainstream voices that dispute the overall importance of either of these.

That includes diverse viewpoints and narratives from Michael Howard's British-Allied The First World War over Gerd Schultze-Rohnhof's German revisionist 1939: Der Krieg, der viele Väter hatte (in which the author cleverly falsifies the history of World War 1 to justify Hitler Germany's activity in the run up of World War 2), and even highly specific sub-histories like Madigan's and Reuveni's recent The Jewish Experience of the First World War.

I would argue that it is no longer controversial to name any of these aspects I counted off earlier as aspects in the run-up of the war, but this was not always the case.

Particularly my own country, Germany, had nationals with a vested and lasting interest well into the 1950s to distribute guilt between the great powers and away from Germany - partially with the financial interest of selling books to Germans who wanted to read what confirmed their beliefs. But it was one of our own who went against this trend and became arguably the single most important historians when it comes to the historiography of the outbreak of World War: Fritz Fischer, with his 1961 Griff nach der Weltmacht, later translated by the author himself into English under the title Germany's Aims in the First World War. The Encyclopedia of Historians and Historical Writings even credits Fischer with causing a paradigm shift in diplomatic historiography as a whole.

Another important reason for the shift in diplomatic history from the 1960s onwards was the influence of the changes that were going on in history as a whole, as the discipline moved away from political history towards social, economic, cultural, and intellectual history. In particular the new history of the Annales school raised a challenge as it was overtly hostile to diplomatic history, regarding it as a nothing more than a history of elites.

This criticism rang home, for diplomatic history was still overwhelmingly concerned with high politics and was based primarily on the documents of foreign ministries and their ambassadors. In order to overcome the limitations of diplomatic history some of its practitioners started to devote attention to the economic, social, cultural, and military factors that influenced foreign policymaking.

The most significant of such scholars was the West German historian Fritz Fischer who, in his book Griff nach der Weltmacht (1961; Germany’s Aims in the First World War, 1967), reopened the debate about the origins of that conflict by demonstrating how Germany’s economic and colonial ambitions had contributed to the outbreak of war. Fischer’s thesis provoked a bitter debate, but away from the specifics of his subject matter his approach showed that to study foreign policy in isolation from domestic factors was a grave error. In addition, a similar influence was exerted by the rise of international relations as a discipline within political science. The application of a social science methodology to relations between states showed that it was possible to move away from complete empiricism towards more theoretical explanations.

[EoHaHW, p. 315]

You ask almost on the side whether or not there is a document that proves that Germany wanted the war or not - and in fact, there are a few juicy snippets in Fischer's book that might be of interest that lay blame at the feet of specifically the German Kaiser Wilhelm II, who, according to Fischer, did not in fact want to prevent a war, but wanted to start a regional war between Austria and Serbia and then stop it from spreading out of the regional frame through other powers' involvement.

That Germany faced the prospect of a general conflagration with open eyes emerges further from an instruction drafted by Radowitz, a Counsellor in the Foreign Ministry, as early as July 7 and sent by Jagow to Lichnowsky in London on the 14th. Jagow warned the ambassador of the possibility of 'general complications';

Germany wished to localise the Austro-Serbian conflict but not to prevent it. On the contrary, Lichnowsky was instructed to mobilise the British press against Serbia, although he must be careful not to give the impression 'that we were egging Austria on to war'.

This epitomised German policy after the Hoyos mission. When Tschirschky had reported on July 10 that Berchtold was now in favour of 'unacceptable demands', but that her 'chief care' would now be how to put these demands, the Emperor commented: 'They had time enough for that', and further made a suggestion which he thought bound to succeed: 'Evacuation of the Sanjak! Then the cat is among the pigeons at once!'

[GAitFWW, p. 56-57]

The Sanjak is a region in the west of contemporary Serbia that for a long time had belonged to the Ottoman Empire, keeping Serbia and Montenegro divided and Serbia from an access to the sea through Montenegro. Kaiser Wilhelm II was part of a German aristocratic class that held the Slavic peoples in very little regard - that is to say being racist against them to a degree that would be funny if it hadn't caused a world war. The 'evacuation of the Sanjak' would have effectively meant an annexation of the region by Austria, marking a clear territorial goal pursued by the German leadership even before the outbreak of hostilities, thus counter-acting the idea that Germany indeed was trying to slow down their Austrian partner from going to war and that, from 25 July 1914 onwards, the emperor was no longer interested in war, as the German Emperor did not rescind his encouragement towards his ambassador in Vienna to drive the Austrians to evacuate the Sanjak in any effective way - at most, he made private remarks, even though those are used by pro-German revisionists to shift blame away from Germany (see Gerhart Binder: Epoche der Entscheidungen, p. 49, as well as Schultze-Rhonhof, p. 44).

It was also Fischer who did among the first in-depth analyses of Bethmann-Hollweg's Septemberprogramm, the German chancellor's plan to annex parts of Belgium and Luxembourg, going against the German pretenses of time-limited military occupations in these small neutral states.

But I suppose I should come back to your initial question - how do so many theories exist? That is almost a question to the historical discipline itself. No historian can ever hope to have a full picture of facts, evidence, opinions, memories and accounts of the era they are researching, even if we don't account for historians with their mind set on malicious falsification and deception. For a long time, there was an active attempt to glorify the history of particular countries and to whitewash their contributions to the war. I wrote at length about the German revisionist scene within German historiography of the two world wars, because this happens to be a field that I have done my part of reading in - but don't think I am just dragging my own country to the cross here. I am sure that there a British, French, Russian and American tendencies to over-justify their own countries' actions towards war time as well, but I am just better studied with the German literature - I'd be glad if one of the other users could pick up the baton for other countries and their own falsifications of the lead-up to World War 1 in popular historiography.

So why would an author aim to falsify history? Well, maybe on one hand, it makes an author feel better to write from within a team with good justifications and the moral high ground (I certainly would feel that way), and on the other hand, it causes less of a controversy and probably increases your book sales if you just tell the average reader that their country is good and the others are bad. With World War 1 in particular, you are looking at human suffering on a massive scale that every human would be glad to not have to feel guilty for - particularly considering that World War 1 arguably, at least partially, caused World War 2 as well, even increasing the amount of suffering that needs to be washed away from any concerned citizen's conscience.

If we look back at Fischer for an example, who dared to go against the German mainstream in a commendable effort, that man was absolutely whacked around by the backlash. There was a clique of nationalist-minded historians around G. Ritter that attempted to discredit him as well as his national allegiance, and Fischer was arguably up against the German historical field as a whole, which in the culture that John Moses dubs Borussianism ('Borrusismus', from Borussia, Latin: Prussia) was dominated by right-leaning if not far-right thinkers that relied on Christian conservative and authoritarian thought structures in their arguments, pointing to a picture of a Germany whose sanctity was not to be questioned. The flying term here is Betriebsunfall, 'work accident', in which the adherents of borussianism claim that the rise of Nazi Germany and World War 2 was not representative of German society or German history, but instead a work accident inside a mostly flawless national history. If, like Fischer attempted, the Betriebsunfall was proven false, this whole picture of a holier-than-thou Germany might collapse.

And it did. Amongst historians, Fischer became probably the single most important contributor to the modern German Vergangenheitsbewältigung, the 'reconciliation with the past'.

So why do historians whitewash the run-up of World War 1? Because it's easy. But maybe they should be more like Fischer and do what's hard.