My speculation: there are a small handful of long dead people, for whom the SSA has never received proof of death, and they fall through some bureaucratic loophole in the "assume people over 115 are dead" rules, so the SSA is still printing them checks. But these checks haven't been deposited in a bank account in many decades, and the SSA doesn't even have a current address to send them, so it just sticks them in a filing cabinet in their basement. No fraud, the government isn't really losing any money since the checks aren't being deposited or cashed, just a little bit of bureaucratic stupidity that no doubt adds up to a rounding error in the federal budget. But you can see how Musk/Trump/et al could spin this to sound a lot worse than it really is.
If you want to find large-scale government waste, it's going to be things like Medicare approving unnecessary treatments, or corrupt officials subtly shaping the bidding process for government contracts to make sure they're awarded to the expected cronies for the expected exorbitant costs, or subsidies that are supposed to go to the poor but are shaped in ways that really cause them to go to landlords or drug companies.
But "we streamlined the bidding process for cloud services contracts to make it feasible for smaller entities to make viable bids" is a non-headline even if it would save billions of dollars. "We're eliminating housing assistance and turning it into a refundable tax credit in the same amount for the same people" would improve government efficiency (and outcomes), but it's not viscerally satisfying and makes people feel bored or uncertain rather than happy or angry.
What sells is simple overt fraud. If you can find some 30 year old cashing their dead grandmother's social security checks, that's a headline that will get people worked up. Which means they want to go find some of that stuff to throw meat to the base, while hopefully also doing those other things that are actually more important but don't get the mob excited.
The opposition then plays politics either way. If you actually reduce Medicare spending on unnecessary treatments you're "cutting Medicare" but if you go find a few clear instances of outrageous fraud then it's "just a drop in the bucket" and you're not making any real difference.
While this is likely true for a majority of cases. I am confident that there are both bank accounts and addresses that have remained valid since the 1940s. For example, ABA routing/account numbers on checks go back to 1910.
Presuming checks are being printed, it would not surprise me if they are being sent through the U.S. postal system. What happens to them after that, though, is anyone's guess. Banks generally stay on top of deceased customers. But even then, a bank account can be transferred to beneficiaries and retain the same account number.
i would expect that banks would flag a check being deposited in the name of a known deceased former account holder. But there have already been cases mentioned in this thread where someone can receive SSA benefits on behalf of a deceased spouse/parent. As such, it is conceivable that those checks could be deposited.