Because it can't be funded properly so long as it relies upon contributions from the workers to the retired. And given how large a portion of the budget it makes up, we can't just use more taxes to fund it.
Why not? We'd only have to add enough taxes to make up the difference, not to fund the whole thing anew. Tax rates are incredibly low right now, the lowest they've basically been in a hundred years, there's plenty we could do to pull in more money if we decide we need to.
In fact, keeping SS solvent is probably one of the few things that would be politically feasible as a reason to raise taxes, something the government should be doing anyway, so in my mind everything about SS (it's effects, it's funding, it's future political ramifications) is a good thing. Why would we want to argue that we should get rid of it and let the old folks rot? What does that accomplish, exactly?
You don't really get it, do you? I'll admit I'm not a fan of the retired population, but that's not what this is about. SS is going to collapse. Period. We can either replace it before the problem spirals out of control, or we can let it spiral out of control, but we most certainly can't keep it. The math fails on a basic level. There cannot be a lower population of contributors than beneficiaries.
No, it's not going to collapse, period. This is just BS misinformation and lies spread by people who want to kill it for political reasons. Social security actually currently shows a slight surplus and is expected to continue to show a surplus till 2021. In 2021 payouts are expected to exceed income - but even if *nothing* is done it will be able to make full payouts till at least 2033 and continue to make 3/4 payouts indefinitely. Now, cutting payments to 3/4 is far from ideal, but it's hardly "collapsing" and 3/4ths is a lot more then "never getting SS payments".
Meanwhile, there are things we could do such as raising the wage limits for example, that would let them make full payments indefinitely, or even just allow more immigration to increase the workforce.
Fun fact: The biggest single cause of the shortfall, contributing 43% is not falling birth rates, or people living longer, but rather the rising income inequality in the US.