But there's only so many dollars you can pull from a wage earned. A dollar pulled for OASDI is a dollar that can't be pulled for anything else, and the person earning said dollar will express dissatisfaction at the ballot box if you try to make up for that by taking another dollar.

A retirement bonus (which is what OASDI is for millions of retired Americans) costs more than a retirement insurance plan, because one gets drawn from by everyone, while the other can simply be denied if it isn't strictly necessary for the person trying to collect the benefit.

A more expensive program requires more tax revenue to administer, revenue that could be used on anything else. Since we haven't had a real national discussion about federal taxes in 30+ years in the US, we now take the spending that would be funded with the tax revenues that could be freed up by a less-expensive implementation of retirement insurance - one that's treated as actual insurance - and instead fund it with deficit spending.