I think your valuing of moves is flawed. Yes, during the fuseki the best move may be worth 20 points. But there are often many moves that may be worth 19 or 18 points. So, playing perfectly only gains a few points compared to playing acceptable moves.
In comparison, tactical situations often have a crucial move that wins many points - in low-level play swings of over a hundred points are not uncommon. There, missing the crucial move can lose the entire game, no matter how perfect the opening was played.