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).

¹) https://people.inf.ethz.ch/wirth/index.html

²) https://people.inf.ethz.ch/wirth/CompilerConstruction/index....

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."

⁴) https://people.inf.ethz.ch/wirth/FPGA-relatedWork/index.html

(edit: And, of course, there's FPGA implementations of Professor Knuth's MMIX (http://mmix.cs.hm.edu/), too: https://github.com/tommythorn/fpgammix)