I dont know. Does your Linux distribution immediately upgrade to the latest kernel when Linus releases a new one?

Besides, the phone software is highly optimised for the specific hardware. It is not a generic software like Windows

Even distros often provide a kernel mainline branch to let you install the latest and greatest kernel in a click.

But there's also nothing to stop you simply building the latest kernel from source and using that - it's pretty easy and it will work fine.

Why though?

Seriously, why can't Android just be installed? Are they building it like the old-timey kernel before modules and embedding drivers in a giant monolith or something ridiculous?