Why?
Other than making rogues cry of course.
Because the current system doesn't make any
sense. Damage is supposed to represent, among other things, how
hard you hit. Even if you're more accurate, being more accurate with more force behind it should still have a greater impact than being that accurate with
less force, and the current system doesn't reflect that at all. As it stands, a rogue thrusts their rapier exactly as hard with Strength 14 as they do with Strength 8, which is ridiculous. If you want to fight in melee, you should need to invest in the melee-fighting stat, which is Strength.
It also leads to weird and counter-intuitive combat encounters, like a Strength 14 air elemental hitting
harder than a Strength 18 water elemental because its large, amorphous pseudopod is better
aimed.
Really, what the current system does is to severely punish any character who doesn't build to extremes. There's no room for someone with above-average Strength
and Dex, because they'll be out-classed in every way by anyone with higher Strength
or higher Dex.
Tht'a pretty easy to do.
Just do it. Dump proficiency and add level to all proficient checks. And add a monster's CR to all their checks.
The game will play identical when facing monsters of close to your level.
You'd need to add it to saves and AC, too, though. As it stands, simply replacing the proficiency bonus with the level bonus in every equation would make it so every attack is successful and only proficient saves have any chance of succeeding whatsoever (I mean, even moreso than is already the case).
And the real benefit of scaling attack and damage would be to reduce hit points. If a level 7 fighter-type enemy has +7 to hit and AC, it wouldn't need
so many hit points that it takes a dozen strikes from a greatsword before dropping. If a giant gorilla demon had +15 to hit and AC, then it wouldn't need hundreds of hit points in order to do its job. It would probably necessitate correcting the damage-dealing capabilities of most characters, as well; a fighter doesn't need four attacks in a round if the demon they're fighting doesn't take twenty hits to drop.