Because having the same DE means there are other many users testing and extending it. Application developers all develop for the same APIs. Bugs from different software configurations are much less likely. I once had to spend an hour getting a functional audio device switcher because the one that came with PopOS permanently defaulted to an unplugged headphone jack, and then debugging when that didn't work. Nobody had run into this particular issue because everyone is using different combinations of critical software. It feels like everything on linux is like this. Time I spend getting audio working is time I could have spent on literally anything else. Tinkering with my computer is not a hobby, but a chore. I choose the OS that gives me the fewest chores.