A quick answer is that people were nomadic for a very long time. When agriculture was adopted and people were staying in one place, systems had to develop for living together.

Agriculture began around 10,000 BCE, relatively shortly before permanent settlements began appearing, such as Jericho.

The short answer is that there isn't any compelling reason to develop agricultural society before we did, but there are a lot of assumptions to unpack in the question.

First of all, the idea that "civilization" only started with agriculture. Archaeologists never or rarely use the term "civilization" anymore since it implies a value judgement. If agricultural societies are "civilization" (hence, "civilized"), then does that by necessity make nomadic societies "uncivilized"? While I personally quite enjoy living an industrial, agricultural society in the present, that doesn't mean it is the only successful way for humans to live.

Agriculture, especially before the immediate past, actually has a lot of downsides to it compared with hunting and gathering, including increased disease transmission (especially of zoonotic diseases), generally longer working hours, and generally poorer diet. That isn't to say agriculture is uniformly worse than hunting and gathering - there are advantages and disadvantages to both - but it is to say that agriculture isn't obviously or inherently superior to hunting and gathering as a lifestyle.

In other words, you can't think of human history as linear, with some sort of end-goal of modern, agricultural society. Everything we did historically, as a species, was based on the information available to us at the time and based on what worked best for us in that particular historical context.

That brings me back to my original point that there wasn't a really compelling reason to develop agriculture before the point we did. Why develop a new method for feeding yourself when existing methods already work very well? Keep in mind that for most of human history population density has been very low. The main advantage of agriculture over hunting and gathering is the ability to feed more people with less land. However, for most of human history, if you started getting too many people to support in the place you are living there was often more open land to move to and exploit. Why develop agriculture when you can much more easily just move to the next valley over?

What changes about 10,000 years ago (when domestication begins more or less - keep in mind that it took a long time to go from the earliest domestication efforts to the city of Ur) is that the Pleistocene (a series of glaciations, or "ice ages") is coming to an end. The end of the Pleistocene sees, generally, an increase in moisture and plant cover across much of the planet. In addition, it seems like human populations are finally reaching densities, at least in some places, where mobility isn't necessarily a solution to every problem of overpopulation or scarce resources. The contributing factors that made agriculture desirable compared to hunting and gathering just didn't exist until around the end of the Pleistocene.

As for evidence of agricultural civilization existing earlier than we think (which you mention in a post below), there is zero reason we should think that we are just missing evidence of earlier agricultural societies. The question comes up with frequency, here and in other subreddits, most recently here.

Isn't this now more appropriate for /r/askanthropology?

This is a question that requires a lot of clarity in terms and concepts. For one thing, what is agriculture? Do you just mean a generalized sense of the cultivation of plants for consumption? In that case, there has been agriculture in a sense for as long as there has been humanity. In contrast to /u/EvanRWT's post (just to let them know I'm discussing it) "simple cultivation" in terms of scattering seeds, replanting stalks, etc is a highly effective and labor efficient way of acquiring consumables. In Papua, for example, many peoples into historical times inhabited the fuzzy border between forager and farmer by manipulating wild growing sago plants, often in ways that are so subtle that observing anthropologists would miss them. In Australia there is evidence of the deliberate use of fire to promote the growth of certain edible plants. Are these agriculture? What about the examples of "proto-agriculture" such as the finds along the Galilee or, more famously, the site of Abu Hureyra, where the inhabitants seemed to heavily rely of deliberately cultivated strands of wild plants?

In general, when archaeologists discuss the development of agriculture, "agriculture" is defined as the deliberate cultivation of genetically domesticated plants such that the population relies on them for subsistence. So to get at your question, we need to ask what exactly agriculture did. The answer, evidence points to quite conclusively, is very little good. The effects on biological wellbeing from agriculture are generally terrible in terms of the development of diseases, increases in malnutrition and starvation, social stratification and more. Agriculture is land efficient when compared to foraging, but requires significantly more labor. So perhaps the best answer to your question is that people didn't become agriculturalists for a long time because being an agriculturalist is not such a great thing.

Beyond that, this is a fairly common topic here, I wrote a bit more here.

As always, Bellwood's First Farmers is a great source.

The best way to explain it is that in those first 190,000 years people were not practicing agriculture but undergoing a series of subsistence changes slowly shifting them towards agriculture. These changes in diet are generally called the 'broad spectrum revolution' and involve as the name suggests a broadening of the diet to include things other than large animals as a significant proportion of the diet - including plants. Depending on the place, the change in diet could have been triggered by 1) Changing climate altering environments and causing declines in large game resources (or increasing small game resources like rabbits) or 2) human overexploitation causing depression of the prey population.

Depending on who you ask, BSR began 100,000+ years ago in some form, but most people now accept BSR leading to agriculture was occurring around 12,000 years ago. In some places this date has been pushed back to 20,000 years (e.g. recent grain and grindstone discoveries in Israel). With agriculture increasingly relied upon from 12,000 on, sedentism was able to develop and from there civilisation as we think of it.