If things are as open as you say they are, where the player's help create the world. Then why wouldn't the DM need to have them in place. What if the PC's wanted to travel to the desert. Wouldn't it behoove the DM to know exactly who lives in the desert? The dangers? The culture? The geography? And then, in the next month they want to seek out those islands off the coast. Shouldn't the DM knows who lives there? Their way of living? Their gods? etc.
Again, if things are so open for these other DM's; no railroading, no forced migration, yet, an open notion in session zero of what exists and what doesn't, don't those two things create a bit of consternation?
I mean, like I said earlier, if you lean super heavily on impromptu, I can dig it. It works. But the other two do not seem to coincide.
As far as your strawman, I am not really sure any person here has argued the way you describe it. And in your said example, PC races plus another dozen is fifteen to eighteen races. That may be too much for certain DM's.