> It is difficult to understand why so many junior programmers use a dozen boolean state variables instead of a state machine
It's not just juniors. The only times I've encountered state machines in the wild are almost exclusively games and virtual machines/interpreters.
The reason, to me at least, is pretty clear. In both games and VMs, you mostly know the rules ahead of time. You have a clear picture of the opcodes you want to implement (add, mul, call, jump, etc.) or the game rules you need.
But for most development, especially with multiple stakeholders or customers and multiple developers working independently, the rules aren't known at the start. In fact, the majority of requirements are brought up months or years after the first line of code is even written. It's like the parable of the blind men and the elephant (https://en.wikipedia.org/wiki/Blind_men_and_an_elephant). Each person, working separately, has a different idea of what this thing is that you're creating. Communication being an incredibly difficult thing, the requirements are never fully rendered coherent by the entire team at the moment where it would benefit them most (i.e. the beginning).