Comment by MobiusHorizons

A single board computer contains an application processor, not a microcontroller. They have very different design goals.

A microcontroller is a type of processor that offers very fast startup, predictable timing and direct access to hardware peripherals. In contrast to application processors that are intended to run an operating system, a microcontroller will not have OoO execution, multiple tiers memory (eg cache and dram). Usually they also lack an mmu and multiple privilege levels of execution (although that seems more common on riscv microcontrollers). All this means microcontrollers aren’t designed to run Linux (even if someone does figure out how to do that) or clock in the gigahertz, but instead provide very predictable low latency execution required for managing real time systems. For example they might need to operate a set of mosfets with microsecond accuracy to efficiently and safely control a motor.