Also, there is the issue of third-party standards.
Zigbee and Z wave are independent third-party alliances Which have created standardized protocols for Home Automation, which makes it much more likely that, say, a light switch from one vendor will be able to work With the hub from another.
You mentioned Wi-Fi: so far, although Wi-Fi has a standardized engineering protocol for the transmission of messages, it has not adopted a standardized home automation protocol for the content of what those messages might be. Each individual device manufacturer makes up their own message content. So a Wi-Fi light switch from one vendor will likely not work with another vendor’s app.