Well DnD consistently doesn't have criticals outside of attack rolls and death saves.
Like the person you replied to asked, what would you even expect to happen on an ability crit? If the DM only lets you roll on things that would be possible for you, then you would succeed on a 20 anyway. If the DM lets you roll on impossible things, then you have a 5% of doing the impossible. Neither option is good.
I absolutely let a 20 or 1 have extra effect whenever it makes sense and feels right. But having it be a core rule would be a PITA.
Not to mention that it would make skill checks even more driven by randomness, which is already a problem.