Why couldn’t they just use usual Linux for that? Why a modified kernel? Is Linux as is not suitable for a phone?

Can’t they just, idk, make a distro? Maybe from scratch? Pop!_OS is working on COSMIC. Can’t they have their Linux-based OS, perhaps with its own things as needed, such as a phone-optimised DE? Or whatever the phone equivalent of a DESKTOP environment would be. A Mobile Environment, perhaps

If my laptop had touch screen with no other method of input built in, and were way smaller, could it not run Linux? Or is that different altogether?