Because if that's the way D&D has always done it then clearly it must be a good thing.
Wait, when was it decided that bickering with the DM about what supplements are legal or what the game will be like before every game was a good thing?
Figuring out what supplements are allowed, and trying to convince the DM to choose a supplement you want is a part of the game. It helps to make sure that everyone is going into the game on the same page. If the DM wants to run a straight up Monty Haul dungeon run campaign, whereas the players are expecting a game involving multi-generational family dynasties and political intrigue, there will be problems.