Are there actually a lot of retirees working as greeters? Maybe you only see the ones who didn't save enough. Or maybe they have enough money but like getting out of the house. Or they're doing it for the health benefits.

I assume retirees prefer greeter jobs because those probably don't require lifting anything heavy, as a stocker or cashier might need to do.

I worked at a grocery store for 5 years. A couple retirees worked there because they had to, but most of them just wanted something to do and health benefits.