Exactly.ability score value is for 3d6 not d20 or we would roll d20, but when they made the game, they realized that bell curve is better for ability distribution
And scores from a bell curve.
There is no neat way to form a bell curve between -1 and +4.
Ability Score handles a lot of values better and simpler than Ability Modifiers.and all of this is also described by ability modifiers.
no need to have two sets of stats for the same thing.
ability scores currently would add too much if you would use them after d20 roll, so just ax them in current form and have ability scores go from -4 to +4 with average of zero(0)
Especially anything above an 8.