WiFi access points have no secret. They use a SSID, but they otherwise offer or know nothing that the client can validate. Every time a device (say, a laptop) connects to a WiFi router, it uses it as if it was a new router and expects nothing in particular from that router. Secrets are more on the client side: the router will require the device to demonstrate knowledge of the WiFi password.
With a SSH server, the server has a private/public key pair, and shows the public key, and the client records that public key. If the client reconnects, then it will require that the server uses the same public key as previously; otherwise, it will complain loudly (as you observe).
There is no such public key in a WiFi router. Clients are not actually interesting in knowing whether the router has changed or not, because the router is only meant to route data to the Internet, not to provide access to some sensitive data or receive sensitive data.
When you talk to your banker, you want to be sure that you have a genuine banker with you, because you will give him your money. When you talk to the cashier at a supermarket you don't care whether he is a real cashier or not, as long as you get to exit the supermarket with the food and security guards don't begin to shout at you.