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.
It is true that Linux-based distributions have this thing in common: the Linux kernel. There have been some GNU/Hurd variants though...
I guess Linus Torvalds and co? First they'd need to standardize a Linux desktop OS.
Also who is paying "Linux" and for what?
Maybe the answer ends up being Valve.
Well at least Microsoft is a platinum member of the Linux Foundation for many years...