Because combat, which is a main component of the game, becomes super repetitive and boring once you hit the level cap and there is no progression.
In Fallout 3, the level cap was a problem in the vanilla game. It absolutely ruined the end game for me once I hit level 20 and still had a bunch of quests to go check out and complete. The game is remarkably less fun when you are just rushing from A to B and there's no longer a challenge when fighting or a reason to continue doing so unless forced by the quest narrative.
Level caps punish people who explore and do everything, unless it's a very high cap that most players will never see.