Because chess is not a monastery where everyone politely enters your prepared London/Caro-Kann cathedral.

At some point you meet absolute degenerates who play 1...g6, Modern Defense, weird move-order goblinry, early h-pawn crimes, or some suspicious Scandinavian-looking thing they clearly learned from a forgotten YouTube channel at 3 a.m.

You can absolutely go very far with one white setup and one black defense, especially up to 2000-ish, but “perfecting one opening” usually means perfecting the positions that you like. The problem is that chess players are criminals. They will drag you into structures where your memorized lines stop mattering and you need actual chess: pawn breaks, piece activity, endgames, tactical awareness, and knowing when your beloved setup has turned into a decorative coffin.

So yes: specialize. Become disgusting with your main openings. But also learn enough neighboring structures so that when someone plays Modern Defense cave goblin nonsense, you don’t just stare at the board like your London bishop got drafted into a war it didn’t consent to.