FrogReaver
The most respectful and polite poster ever
If everything scales, nothing scales. If both you chance to succeed and your target both go up by the same number, there's no relative change. It's like as if there's no scaling.
So if everything scaled, you wouldn't get better at anything when you go up in level. Adding +6 to attack and +6 to AC would just wash. Adding +6 to your save DC and +6 to all saves would be a wash. Adding proficiency to targets has the practical effect of doing the opposite of the goal of getting better.
Giving two saves out of six gives some differentiation between classes and puts more tactics into spell selection.
As for things like non-proficient skills and non-proficient weapons, that's how they kept it meaningful to be proficient or not. You only add your Proficiency bonus to things you are Proficient in. Differentiation where people get to stay good at what they train in. There would effectively be no skill system if your non-proficient skill got your proficiency bonus because your choices would be meaningless. It would all be ability score plus proficiency.
Right. Scaling in this context is just the level based scaling. The bonuses for You and the npc of the same level as you will just cancel out so effectively no advantage. But for different level foes it’s a big deal.
Of course this must be built upon a system where characters that are good at say swinging a long sword like a fighter get some additional benefit to swinging the long sword over a wizard of the same level