Wilhelm already was a king: the King of Prussia. The point of the imperial title was to elevate its holder above other kings. Thus by being an Emperor, the Kaiser could elevate himself above the other kingdoms which constituted Germany (Bavaria, Saxony, Württemberg). If he were simply a king and not also an emperor, he would have merely been the equal of those figures.

The Austrian imperial title was to some degree a continuation of the imperial title of the Holy Roman Empire. As that empire was coming to an end, the Austrian rulers were in need of another imperial title in order to maintain their 'rank'. It also gave the Habsburg Monarchy an overarching structure, whereas it had previously been merely a collection of titles.

I'm not sure if your question is completely accurate. Queen Victoria, for example, was "Queen" to Britain, but "Empress" to India. If we were to take her case, are you trying to ask why she was not "Empress" to Britain also?

So you have the answer here but all a bit mixed up, let me clarify. When England took over other Kingdoms their kings were removed from their thrones and there was only one King (or Queen) who ruled say England and Scotland as the United Kingdom.

In Germany when the German Empire was proclaimed in 1871 there were still four German Kingdoms that made up the empire, along with other assorted territories like ducies and principalities etc. Those Kingdoms still had Kings and to rule over the kings there had to be a higher ruler, the Emperor William (Wilhelm) I.

Victoria was the Queen of the United Kingdom of Great Britain and Ireland and (from 1876) the Empress of India due to there been princely states in India. Interesting there is an argument that Queen Victoria wanted to be Empress due to the fact that her daughter (confusiningly also called Victoria) was married to Wilhelm I and so when he became Emperor his wife became Empress and so the daughter outranked the mother, until five years later she too became Empress Victoria.

To answer the other part of your question, Wilhelm whilst been the Emperor of the German Empire he was still also at the same time King of Prussia - one of the four Kingdoms that were a part of the Empire.

Franz Joseph was simultaeously,

Emperor of Austria, Apostolic King of Hungary, King of Bohemia, of Dalmatia, of Croatia, of Slavonia, of Galicia, of Lodomeria, and of Illyria Plus many more titles.

The title of Emperor had symbolic significance. It was a bit declasse to simply invent Imperium out of whole cloth.

The Austrian Emperor had his title theoretically as the successor to the dissolved Holy Roman Empire (and Napoleon had assumed his Imperial title specifically as an Age-of-Enlightenment challenge to "old Europe" as it were). The Russian Emperors had theirs as theoretical Orthodox successors to the Byzantine Empire. The German Imperial title was adopted specifically as a parallel to the old Holy Roman imperial title, given that the German Empire was seen as a reunification of the all the disparate German states into one nation.

Victoria only assumed the title of Empress of India in the late 1870s after Britain dissolved the last vestiges of the Mughal Empire in the aftermath of the 1857 Indian rebellion. It wasn't wholly popular in the UK- there's a cartoon from Punch which shows Disraeli dressed as a Middle Eastern trader offering her an overblown Oriental crown.

This is why Victoria and her successors were only Empress in India. Plain Tales from the South China Sea (an oral history collection of reminiscences from the last generation of British administrators in what was then British Malaya and the Straits Settlements) actually makes a point to state that at mess nights and such they toasted the monarch as the King or Queen, never the King-Emperor or Queen-Empress, as was the custom in India.

The British Imperial title was specifically an Indian one- Victoria and her heirs derived imperium from the Mughal padishahs and the title was irrelevant in their other dominions.