Actually, as of last year and for this year (and probably for the future lest we face a tax hike which would actually just be a return to normal), workers pay in 4.2% of their earnings. However, they get credit in the future as if they'd paid in the full 6.2%. That was the part of the bill that pissed me off the most; it's just a big middle finger to future generations. Congress didn't have the balls to say if you pay in less you get credit for less in the future, they'll just leave it to future generations to try and figure out how to pay for it.
And in the "no good deed goes unpunished" department, some states allow FICA paid to be an itemized deduction, and since it dropped, their state tax bill went up. I had several clients who never owed Alabama until this year.
I forget why the earnings were capped. I think because benefits are capped. But if taxing all earnings would eliminate social security's debt, it should be looked at at least.