Attributes

One of my players suggested that attributes didn't make any sense. I remembered one of the tricks for using 3 six-sided die to determine stuff like strength, dexterity, intelligence ...

The idea is to answer the question: could an average human do ___ action? All they have to do is roll their attribute or below. Someone with score 10 (an average person) has a 50/50 chance while a person with a score 20 has a 100% chance. As much sense as that makes, that doesn't come up in D&D.

So I guess I'm not really sure why that system is still used.