Because we can't install that on phones and even if we did, we need to use Android apps to do basic daily things.

Phones are not like PCs, you can't "just install a different OS". You also can't just build a phone from parts like you can with a PC, it comes locked in with the OS, with proprietary drivers and advanced cryptographic DRM measures.

And even if we did get things to the level of desktop Linux, we can't run any of the apps we need for everyday life. Most of these things on desktop are web-based, so you can use them on Linux, but this isn't the case for mobile and many things only come in mobile. Bank apps, government services, digital identification, mandatory companion apps for other devices...

If nothing else, we need to keep Android as open as possible because it makes it easier to port those things to other platforms and maybe one day have a proper alternative.

Oh, and it's not like we have a good alternative. The current Linux stack is completely inadequate for mobile use. An average phone has something like 50 apps the need to be able to react to any of a few dozen different local or remote events at any moment, yet also need to use approximately zero CPU cycles to do that. We need a brand new app paradigm if we want mobile Linux to succeed and it's not looking like that's going to happen any time soon.