They don't need to be foolproof to be successful
Like those web sites that try to trap you on their page claiming you have been infecting with malware or that they're the FBI and have detected child porn on your PC. You can't get off them by closing the tab or whatever, but you can if you simply close your browser and uncheck that page when restarting it, or restart without your old tabs. Not everyone knows to do that though, and such pages wouldn't exist if they weren't successful in fooling some people.