Ultimately the cap is because there is a max on how much you can receive per month. So they align with each other. But honestly if you're at the point where you're hitting the social security cap, then it's not even going to be your primary source of retirement. In which case capping benefits but not capping contributions would hardly be noticeable, but would help keep social security solvent.
To be clear, a maximum on monthly benefit, not total!
What?! How in the world did you get that out of what I said.
There is a maximum monthly benefit. Maybe I need to go back and reword it or something, because this totally misses my point.