Shifts from early game to mid/late game?

I've noticed in many balancing changes that there's been a lot of power shifts from early game to mid/late game. Why? I always understood there were champions who are very strong early game but then fell off late game and vice versa. But with every patch, there seems to be some change that weakens a champion's early game in favor of their late game. Why? Why is Riot shifting everyone's power to late game?