Because of capitalism & companies that dont want your phone to have a life after use.

That said, some do. Most Motorola devices can have the boot loader unlocked, and all Google Pixel (and nexus) devices can be unlocked.

The next problem is the closed source BLOBS from companies like Qualcomm who make the majority of the ARM cpus in these phones. Even though they are ARM they are not standard enough to run without some propitiatory code that is obscured and encoded into the firmware.