There's lots of simple things to do to address Social Security's budget, including modest raises in retirement age, means-testing benefits, and changing the way we collect FICA taxes (FICA is itself a flat tax, which is kind of silly, and the withholding cap is a travesty).
for sure. it's just that removing the cap is simple to understand and legislate, and is widely supported--around 80% of all americans. unfortunately that 80% doesn't include the majority of congress critters.