Making wheat into an edible item takes a much greater amount of effort (threshing, chaffing, grinding, baking if for bread) than other crops do. What made early humans put all that effort into purposely cultivating a grass rather than focusing on vegetables?
How did grains go from tiny little low nutrition grasses to such a dominant part of humans diet?
Because grains are a good food that provides a lot of calories and a reasonable protein/nutrient profile on a small unit of land.
First, let's get rid of the premise that grains are near worthless as wild plants and were only eaten after the origin of agriculture. Grain consumption goes back tens of thousands of years 1 2 3, long predating agriculture. This makes sense: people wouldn't domesticate a plant they weren't already familiar with. Thick stands of wild grains are pretty unfamiliar to us in the modern world, but that's because farmers have long ago plowed under most of the places wild grains grow easily. Although it's worth noting that wild rice is still collected today in the upper midwest 1, and note that the people in the story linked collected 80lbs in a single trip. Even wild grains can grow in thick stands allowing for rapid collection of large quantities.
So why cultivate grains? It can't be because of taxation. The domestication of wheat began between 10500 and 9500 BP 1 predating the origin of the first large states and taxation by thousands of years. By the time the taxman rolled around, wheat and other grains were well established. Perhaps grain made taxing practical, rather than the opposite being true.
Agriculture in general has an origins issue. As others have noted it's not particularly energy efficient to farm food, since you have to put in the labor to plant and tend the crop rather than just coming along to pick the resulting produce. While there are many hypotheses about the origin of agriculture, many emphasize the space or density efficiency of it. You may need to work harder to grow crops, but you can get many more calories per unit area than the average forager. I don't think anyone has a really conclusive reason for the origin of agriculture.
But as for why wheat, that's a bit easier. I was unable to find good numbers for neolithic crop yields, but more recent estimations of calorie-per-acre crop production put the grains much higher than most vegetables 1 2...with a couple of important exceptions. First are sweet potatoes and potatoes, neither of which were present in the ancient near east. Second are various legumes, which were widely cultivated as a complimentary crop to wheat and other grains 1, and finally there's maize, an exceptional new world grain that required quite a bit of selective breeding to reach its current massive productivity. In general, staple crops just blow vegetables out of the water in term of calorie per acre, which is no doubt why even today we get most of our calories from a handful of staples.
But wheat and other grains have another advantage over most other crops: as a seed with low water content, it keeps very well compared to most foods. It requires a bit of processing for easy digestion, but that very fact processing is needed because it exists in a stable form not as easily destroyed by mold or insects as your average vegetable. You can't easily store your brassicas or tomatoes or fruits year-round, and thus live off them in the off-season. But you can store your grains.
Hi OP, this question on "early humans", the practicality of grain in the human diet, and the development of agriculture would be worth x-posting to our sister sub, /r/AskAnthropology.