Because the nuance of calling a PC a PC is that it supports a standard motherboard configuration and any Linux kernel will be able to boot. Mobile devices aren’t like this and every component requires a specialized driver to work.
And just to add to that, anyone who has installed Linux before knows that everything isn’t guaranteed to just work out of the box. Laptop WiFi cards, for example, frequently have driver issues. So it’s not quite as simple as OP is saying, even with PCs.
Because the nuance of calling a PC a PC is that it supports a standard motherboard configuration and any Linux kernel will be able to boot. Mobile devices aren’t like this and every component requires a specialized driver to work.
And just to add to that, anyone who has installed Linux before knows that everything isn’t guaranteed to just work out of the box. Laptop WiFi cards, for example, frequently have driver issues. So it’s not quite as simple as OP is saying, even with PCs.