If a pitcher can pitch better in a short inning, why don't they use 6-9 pitchers in one game (without SP)?

It's well known that any starting pitcher cannot pitch at full velocity for all pitches. If he does so, he doesn't last 5-7 innings. If a winning of the game is the ultimate goal, and if we ignore the ego of the starting pitcher, isn't it better to replace Starting pitcher with many many reliever/closers? Or did a team like Oakland A's try this before?