Why is enemy-scaling such a common thing in video games?

It got fucking unbearable in borderlands 2 it was in 1 I'm pretty sure but it wasent as bad (although it's being a lot longer since I played 1)

Because it's easier than making actual challenges in any sort of game - be it RPG, action, FPS, etc - where the player character's power progresses. You essentially nullify any need to actually balance progression of player skill and challenge, effectively killing any meaningful progression of skill - even in games where the primary skill is managing the progression of a character like RPGs.

Scaling is usually done for some model system, and if you'll optimize better, you can get ahead of the curve.