Cellphones have a lot of peculiar, absolutely undocumented and very buggy peripherals, and tricky system-level power management demands.
Pi's SOC was meant to be designed in to a variety of products, so needed to be documented and understood. Cell phone chips are dumpster fires, as a rule, and are forgotten by their manufacturer within months of first delivery.
Good points, but on the other hand the market penetration is enormous, e.g. Samsung probably shipped hundreds of millions of their galaxies.
There’re thousands of exceptionally good software developers worldwide. It’s surprising no one so far found a good way to re-purpose the hardware. The OS kernel is open source already (wasn’t the case for the original xbox), and all these proprietary firmware blobs are shipped with the devices.
The blobs are compatible only with a particular, ancient and very heavily patched kernel version. Most of the important code is entirely inaccessible in the "baseband processor" that actually operates the radio hardware, and really owns the whole phone. And solving one phone model would barely help with the next one, if at all.
Pi's SOC was meant to be designed in to a variety of products, so needed to be documented and understood. Cell phone chips are dumpster fires, as a rule, and are forgotten by their manufacturer within months of first delivery.