Of course, they can't test their own code manually. That's two roles: developer and QA!
So a backend developer is at least 3 roles of work. Are you making 3x the salary you should be?
The biggest personal reason generalist roles should be priced higher is the time you spent to learn multiple things well enough to get a job doing them. If you accept a role as full stack that pays the same as a backend only role, you're essentially devaluing your own time. The other perks are reducing head count and giving the business that extra flexibility and convenience. If your salary doesn't reflect that, you're giving it away for free and we know how much businesses make us pay as consumers for convenience.
It might seems strange to consider a lot of factors, but you have to remember that generalists can bring quite a lot to the table.