Bcs nobody is monetising it that hard from the ecosystem-monopoly pov.

And "standardised" prob isn't the best word to use imho.

Linux offers much better backwards & current compatibility that the other two/three just do not. Saying 'it's not supported' does "standardise" things much quicker.

Also there are diffident distros by different people or companies - a bit like saying how Windows & MacOS arent standardised and look/operate differently.
But you can make your own Linux distro or modify it's kernel or window/packet/etc manager all you wish.

Also the point about how Windows and Android keeps changing stupid shit for no reason (un-standardising the UI experience though time) but an average user like my father prob didn't even know when his Debian got upgraded (even between distros he didn't notice that much, now I have him on a rolling distro & it's even more seamless tho others basically do the same).

Oh, and if by 'standardised' you mean the look & feel ... well thats for nerds and power users, people like to optimise stuff for themselves. A bit like car seats where one fixed seat won't fit all.