"Since forever ago, I've wanted to try writing assembly, even if just to understand why the Rollercoaster Tycoon creator would write 99% of the game in it. To be fair, even after all of this, I still don't understand why they did that."
Hubris?
Hubris?
Frankly, depending on their experience, assembly may well be familiar, efficient and effective.
Some people who got going early on in games, in the 8 bit era, went right for assembly language because that is where the speed necessary to make magic happen on the screen was.
Action / arcade games pretty much demanded it, unless they sharply limited what happens on screen.
You might find some of the early stories informative. Nasir, for example, did his first dozen or so games with the mini line assembler in his Apple 2. No source code, no labels, etc...
On some of the projects I did on 8 bit computers, it was all about assembly. One planned out a few things, worked around where the screen was, or needed to be (depending on hardware capability), on screen graphics, other data needed, and then build in chunks, saving stuff off to disk.
Every so often, load several in, tie them together, save that off, then run it. It is a different style of thinking for sure.
I bet that developer knew how to build something up that way and went with what worked, was performant, and that was probably the easiest path for them.