Even if this was overcome (checkra1n) or they provided for bootloader unlocks to run an unsigned firmware, you would still then need to reverse engineer all the drivers. It’s why you don’t see stuff like the original iphone SE’s running different OS.
An Android port to iPhone hardware does actually exist (for iPhone 7): https://projectsandcastle.org/
Obviously not everything works, but someone did try.
It would be difficult, but it would at least be possible. See projects like Asahi Linux where they did the work to reverse engineer the drivers for ARM-based Macs.