Good question! The point is to set a limit to when you can start running. The offside rule doesn’t penalise you for outrunning your opponent, but for starting to run too early.

The point is that, until the ball is played, you can’t go behind the defenders. Specially, this prevents attackers from just lurking around near the opponents goal.

Why do we want to prevent that you might ask? It turns out that if you’re allowed to just hang around the opponents goal and wait for the ball, it’s a superior tactic to do so. However, it also makes the game much less dynamic, and typically leads to teams just lugging the ball across the field, hoping their attacker gets it. This is a lot less fun to both watch (for obvious reasons) and play, because as a striker your forced to spend most of your time not being involved in the game, and as a defender your forced to spend most of your time just following around the striker way back in your own half.

In short, without the offside rule, the game becomes much less fun, because it forces you to spend a lot of time and energy on a tactic that amounts to “constantly run away from the defender/chase the attacker”. Having the offside rule means you actually have to outplay or outrun your opponent in order to score.