Don't care or don't know? Lets face it how many average consumers know how to network?
A large number need to pay shops such as PC World for an installation service to take it out of the box, plug the power cable into the mains, plug the monitor into the pc, press the on switch, then follow the guided instructions to start it! 2 plugs and a button and they can't manage it.
In such a world, do you really expect these same people to network a camera? I guess most google camera networking or see it in a manual on the home cameras, and out of the option see Port Forwarding as the easy option then go for it without any knowledge or care of the consequences. Even where they are aware of the threat, most are probably looking for ease as they don't understand VPN, and think no hacker will be interested in them, so take the risk as they simply want to get it set up and working.