Social Security is not an expense of the government. Well, for the most part. It’s a pension plan that people pay into. Similarly for Medicare. People pay into it before retirement. Then at a certain age they get this insurance benefit. Private insurers will not insure the elderly. If anything you should characterize this as a private insurance industry subsidy. It allows insurers to offload their riskiest clients to another entity.

Your characterization is badly wrong.

EDIT: Until recently Social Security took in more money than it paid out. It’s run by the government so its expenditures are counted as a budget item. But it is a sound pension system whose defects can be easily fixed if a certain party would actually govern. It is not the largest expense by the government. It’s just that it is a government run pension system. It’s not an entitlement either.