Wifi congestion for one. The other issue is a lot of people don't want these devices on the Internet. Don't want them phoning home with your personal data. Don't want them to stop working just because your ISP sucks. Z-wave, Zigbee, and Threads/Matter keeps it local, is faster, and is more reliable.
Zigbee mostly uses 2.4Ghz, so it's not helping remove congestion from that band anyways but I guess the other protocols do. Can't the devices phone home as soon as they're connected to a hub that's internet connected? Even if the hub has to cooperate with the device, they're made by the same manufacturers so I wouldn't trust tleither of them.
With wifi I can spin up a separate iot vlan that cannot access the internet. That vlan doesn't require my ISP, it's entirely local. I get to control exactly who connects and even who they connect with. I don't see that same control with the alternatives.
I guess I do see an argument for very low power devices using a lower power protocol, but I guess I just don't have any of those devices so it hasn't been an issue for me. And like you said traffic congestion is a valid problem, I've just never experienced it.
ZigBee devices are often able to be used with a 3rd party hub. For instance, all the IKEA stuff works with any standard ZigBee hub. They don't have a line to the internet if you control the hub.
And a lot of hubs are USB sticks that plug into your home automation system. They'd have to navigate that system to get to the internet. Not impossible, but highly unlikely.