It took a lot of work to get PC parts to become interoperable. There’s all kinds of special negotiations that happen at boot to discover the state of hardware. There are standard drivers for most peripherals
Mobile devices aren’t expected to change hardware so everything is hard-coded into the ROM. No discovery protocols are supported. Standardized drivers make it hard to ship new features or squeeze every last bit of performance so no effort goes into making them
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.