[I'm well aware of who I am replying to here, I'm merely taking the bait and providing some hopefully fun links for people to dig into. :)]
Because that's how Professor Wirth ¹) does it ²), because it allows for some interesting educational side-treks ³), and because the difference between a fictional and real microprocessor is smaller than some might think ⁴).
³) "So what instruction would you add to this CPU to make the work we've been doing easier? Great, implement that instruction in the provided emulator."
Yeah, well, I learned on a real 8 bit microprocessor (6800). It has about 40 instructions, all are simple. It's quite a dopamine hit to run your first program on an embedded system and see it working. It felt like being Master of the Universe!
(FPGAs did not appear for another decade.)