WHY is there a cap for achievements?

For example, after you get 20/20 in an achievement, it should keep going up to 21/20 and so forth..

Why should there be a cap?

The sky should be the limit.

I should have about 50/20 in Super Sleuth, 30/20 in Do No Harm and 40/20 in Scumhunter. SMH.