Haha, no: It's because if you put an MMU in a microcontroller it's not a microcontroller anymore! That makes it an "application processor" of which there are

Haha, no: It's because if you put an MMU in a microcontroller it's not a microcontroller anymore! That makes it an "application processor" of which there are many tens of thousands to choose from these days. There's nothing stopping you from taking say, a Raspberry Pi 4 and using it as a bare metal device (no OS) just like an Arduino. Or taking that same chip (BCM2711) and putting it in your own board to do something similar. It just wouldn't be economical for that sort of purpose most of the time.