Because then we could just have a message that pops up when you first join the server and then make it so you have to enter something from the rules in order to start playing the game.
I'd find this more than annoying if I were a player. People want to get into a server and play it. Having to go look up the rules, find the special one the prompt wants you to input seems like a waste of the player's time. It wouldn't enlighten players on the rules, just finding the certain rule the said prompt demands so you can get into the server. Makes it into a chore.
Realized it said first time you join the server, not every. Above still applies for the most part and the rules can still change. Long-time players won't care that much, newer players will obviously get stuck, etc etc. Some addon out there gives each player a small text box when they join. Wouldn't be able to fit all the rules in it since they're too long. I'm under the impression that if a player doesn't read the rules, the fault is on them. Adding something like this in would just make more work for other players, or admins who may not be experienced with it, attempting to help others somehow getting stuck.