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
Also, a lot of people are not aware that the PC world was even more wild west before this standardization than the current state of smartphone.
I do hope this standardization would come soon. You'd with all this collection knowledge the humanity has, people would come up with a more uniform solution...
The problem here isn’t knowledge, but incentives. Like, someone could design and start manufacturing a phone with very standard stuff, but it wouldn’t sell except for a dozen enthusiasts. Even on PC, Linux isn’t as widely used as Windows on consumer hardware, as there’s no focus on user experience. For a phone like that to work, they would need to solve a problem most other phones don’t solve.
PC standardization took a few decades but there were not many adopters initially (compared to the total population) - mobile is different, basically everyone everywhere has a mobile device, so standardization will likely be much faster