There are also many examples of similar things where, in order to get something to work, you must either manually port-forward or use UPNP. And most folks wouldn't even know how to find the port forwarding settings on their router, they just want to play their network game they just bought for Christmas or use their media server they just set up or whatever. So the option to "turn off UPNP and use port forwarding instead" is not viable for anyone except the experts.

Some people see advice on the internet which says "turn off UPNP on your router to increase security", they do so, and then their toys stop working. So they turn it back on and leave it.