If you play a racing game the game doesn't really have an upper bound in terms of skill or performance: you can always drive a faster lap. For many other games that isn't necessarily the case. For example my favorite game is Hitman: Blood Money, and once you beat all the levels with Silent Assassin rating on Professional difficulty you have maxed out the game as far as it is concerned. If you have already done that many times, doing so again ceases to be a challenge or source of accomplishment.

Speedrunning is one way (but not the only one by any means) to make a bounded game an unbounded one.