I wouldn't say that it is "because of the Maillard reaction", or at least not in the strict either-or sense you seem to assume in the question.
There are thousands of reactions which happen while cooking, and only some dozens of them belong to the Maillard family. And certainly a larger variety of reactions happens when you have more ingredients in the pot - simply because you have a larger variety of inputs to react with each other.
Thousands of years of observation have found out that this results in tastier meals with less effort. First, it increases the complexity of the taste you create. Second, you want to brown the other ingredients too, not just the meat. Given the choices of 1) brown the meat and not the others, 2) brown each ingredient separately, and 3) brown all ingredients in the same pan at the same time, you will find out that the third option is the easiest one logistically, and gives you a somewhat better taste than the second, and a hugely better one than the first. This is why cooks do it that way.
The kernel of truth in your assumption is that indeed, Maillard reactions happen there. They happen both when you cook the meat alone, and when you cook it with other ingredients. It is even plausible that different reactions happen when you cook it alone than with other stuff (e.g. meat protein reacts with meat sugars, vs. meat protein reacts with onion sugars). But that's only a tiny fraction of what happens when you cook the food. So I would not declare it "the reason". The reason is the total combination of all these not-yet-researched reactions that happen when you brown meat and vegetables and seasonings together, which chefs like to think of as the art and magic of cooking.