Because the game's design does not in any way accommodate it. If you look at the DCs for various tasks, they anticipate averages obtained without Advantage. Allowing Help to be highly prevalent allows the players to reduce Easy DCs to Trivial, Medium to Easy, etc., allowing the characters to operate a tier higher than their level indicates. You can do that, if you like, but it trivializes out-of-combat content of their level. Like anything in TRPGs, you can just DM around that problem... but why force yourself to do that?

From a DMing and storytelling standpoint, I also think it's rubbish. People play D&D for the risk and variation; the fact that every action has a 5% chance of detonating in your face is what makes the system so fun. Players always want to reduce risk, because they want to win, but that's not what's best for them! They think they want less risk, but they should want more - setbacks are what make for memorable stories! If you let them reduce the chance of critical failure to .25%, you're doing them a massive disservice, and making your game much less interesting.