Am I the last person to use them? See a lot of youtubers who use regular sockets on impact guns. What am I missing?
You're missing that they are either too poor to own a set with the size/dimension they need, or too lazy/ignorant to bother getting the proper tool for the job... or they just don't care.
Same could be said about using a 12pt if there is no issue getting a 6pt on the fastener.
At the same time, I'll chance using around ~12mm or smaller on my 1/4" impact driver, because fasteners of that size or smaller, don't usually need much torque to remove, even if rusted, and the impact driver itself, and the 1/4" hex adapter, are both torque limiters.
I don't ever use non-impact rated sockets on my impact wrench. First thing I did after buying the wrench was buy a socket set for it, mostly using sizes between 15mm and 32mm. When I need one of those sizes, I am more likely to use an impact socket on a hand wrench than a non-impact socket on an impact wrench.
IMO the larger risk isn't sockets *only* breaking per se, but that if chrome plated, when it does break, the plating can flare out and be like a spinning razor blade. This is why impact sockets aren't just more malleable metal, but also not chrome plated.