Because Zachtronics games are constrained in ways that real ISAs aren't for the sake of good puzzle gameplay. It's about as meaningful as trying to learn to be an infantryman by playing Doom.
I came to hacker news to take a break from a TIS-100 session, and read this comment. It frustrates me that the TIS-100 machine does not use real bytes. I have been working on puzzles that require division, and am sore that there is no right-shift.