It's hard like abstract algebra is hard. When things are simple, you have fewer tools you can reach for when trying to figure out all the ramifications of your few rules. Simultaneously, when you reach for examples you need to keep straight what properties of your chosen examples are due to them being instances of the abstraction at hand, and which are (from your current perspective) noise.

That definitely doesn't make it "not simple" - it's the simplicity that's the trouble :-P