Joking aside, I often hear people say "they should" when talking about GNU/Linux (for example: "they should just standardize on one audio stack"), as if there were a central authority making those decisions. What many don't realize is that with FOSS comes freedom of choice... and inevitably, an abundance of choice. That diversity isn't a flaw, it's a consequence of how the ecosystem works.

There's free choice for those OSes to use different kernels, but they don't, they all use the same Linux (rather than say BSD). There's a lot of advantage in getting aligned on things, even though anyone can choose not to.