Why do AD scaling abilities fall off mid-late game?

Any hero with a strong emphasis on ad scaling abilities rather than autoattacks seems to spiral downwards in damage as the game progresses and snowballing only slows this process down. Heroes that come to mind that are good examples of this are Urgot, Pantheon, Renekton, Ezreal, and so on. On the other hand, AP scaling abilities tend to hold their strength quite well at all stages of the game. Why is this? Are all heroes designed intentionally with higher scaling armor than magic resistance so you need to benefit from stats like crit and attack speed in order to overcome the scaling armor after a certain point in the game?