Earlier quoted context omitted.
It does, however, require supporting it, and those are engineering resources that could be spent elsewhere.
Software developers also often forget that UX is a thing. "Just add feature X, Y & Z". This is why you never let a programmer design UIs, you end up with a mess with dozens of buttons, menus and submenus everywhere. Every new feature comes at a UX cost, and in this case, the cost of the UX outweighs the benefits of you micromanaging your memory footprint.
That's not exactly an easy problem.
Currently used apps
scheduled background work
currently active foreground services like the navigation app you are curently using, the image that is uploading in the background or the music that is playing .. stuff without a visible UI (except for a notification so you know what is going on). All important but not the main thing displayed right now.
You also need to throttle the CPU when the phone runs too hot.
But not run it at minimum speed either because it would make everything laggy.
Also, if you pick up your phone out of your pocket, it automatically wakes up and increase CPU frequency because it is likely you are going to use it.
Etc ..
This is very complex problem, it is ok to let a machine balance this for us.