To me, a significant part of the commonality is the preciseness of the text, and how much of an impact that can make. Many legal cases hinge on the exact phrasing of a law, and sometimes you don't realize the choice of a word is significant.
Another part is that it's hard to program them in a sane way because the rules are arbitrary. It's possible to code how the rules are, but it's impossible to guess how they'll change in the future. Two things you assumed would never interact might need to interact in the future. Two things you thought would never stop interacting might suddenly stop.
I'm not an accountant, but in the Magic world, you would assume that the turns go in a set order, right? Nope, there are cards that reverse the order of turns permanently. You would assume that a player always controls their own turn though. They don't though, there are cards to let you take someone else's turn. There are also cards that let you play the cards out of other peoples' hands.
I looked at coding it, and it turned into spaghetti pretty quickly. Everything has to have ways to interact with everything else, and testing becomes incredibly difficult because of the sheer number of ways these things can interact. And it's hard to make even the most basic of assumptions because they're liable to change at any point. There are no "laws of physics" that at least define a set of possibilities.