Because, and I say that having been a mod on several sites, a rules list that short on a popular site *will*, at some point, invite people who will whinge and whine because they did something that doesn't neatly fit into those rules, or will argue subjectivity ad infinitum, or will otherwise find ways to do what they want without regard for petty concepts as "order" or "other players' enjoyment of the site", and you eventually get 4chan. If you're lucky, you get [one of the better boards]. If not, you get /b/. Without the "best buds" part of the vitriolic best? buds/frenemies dynamic. It hasn't happened with your site because your site (apologies to your ego) isn't big or popular, but, assuming it doesn't peter out like most such boards do, it will. You already had to go from it being rules-free to needing rules, if that Ex Post Facto clause is any indication, and it'll only get worse.
The thing you and others like you often fail to grasp is that rules are typically not made just for the sake of making rules, and administrators who already have full plates are not going to add to their workload unnecessarily just to tick you off or be spoilsports. The rules are extensive because they *need* to be. 9 times out of 10, when you see a rule, it's because someone, somewhere, tried it, and the thing being banned was detrimental to the site, either by a single instance creating a shitstorm, or by the Monkey See Monkey Do effect where one person getting away with it results in more people doing it, making what was a small issue into a huge mess.