Also those ability score improvements can't go above 20. This is done to keep bounded accuracy, which is a very good thing. You could create some utterly broken chars in d&d 3.x, like chars with ridiculous high AC, this made it impossible for normal monsters to hit them(except crits), if you adapted the monster to those OP builds did the game become impossible for non abusive builds.
There are however bonuses to go above hardcaps, such as the boons per XP above level 20
