Most class features don't need to.
Using lv1 barbarian rage gives you +2 attack/damage at any level. Using a magically generated +1 weapon instead of a bought weapon grants you +1 attack/damage at lv1, and more than -4 attack/damage at lv20. If it scaled by level though, as greater magic weapon does, you would consistently get a bonus.
Equipment scales by level by default, so replacing it with a fixed bonus is effectively negative scaling.