[GrilledChips]

Decoupling. The raspi pin header is a defacto standard. The raspi foundation have to maintain a very high degree of compatibility with it. Have a look: https://raw.githubusercontent.com/pinout-xyz/Pinout.xyz/mast...

See all those various overlapping functions? It's the combination of functions on every pin that has to be mimicked too.

Even though the raspi foundation get a fair degree of customization from broadcom, putting peripherals on a chip gets harder for process reasons as you push for performance. This is the same reason why the M1/M2 implement the eUSB spec and have external eUSB repeaters, instead of putting it fully on-chip. The foundation clearly want to go in the direction of increased performance, so it makes sense to push all that messy GPIO stuff off onto a custom chip, get additional cost reduction from including ethernet and USB (fewer chips), and also continue to build their chip design acumen.

I wouldn't be surprised if arduino go this way too. They're already severely limiting themselves in performance and ram by selecting a 5V ARM chip, and matching the exact peripheral combination of the atmega328 is just as much of a millstone around their neck too. They could go with a much cheaper and faster arm core communicating with an AVR chip to manage the IO pins and have a significantly more capable board. Lots of 3rd party AVR+ESP32 boards already do this.