Why are there no wireless switches?

Switches are intelligent networking devices that take signals in from a particular port and then rebroadcast it on a specific port that it knows has previously hosted the intended recipient. In the event that the port is not connected or a different device is connected then it rebroadcast on all ports until it learns where the recipient is.

A hub on the other hand always broadcasts on all ports.

Wireless networks do not have an equivalent of wired ports and (theoretically speaking) are equivalent to 10base2 coaxial style network cable where signals are broadcast onto a wire that connects all machines at once. Hence a switch or even a hub is not a relevant comparison. (There is also link encryption but can be ignored for the moment)

Back in the day in order to traverse network segments you needed an intermediate device which watched for signals on one network segment that were meant for the other and rebroadcasted back and forth as necessary. That device was a router. It is still what your current router does, it takes signals from wireless, wired, and WAN networks and moves them as necessary. Routers operated at a higher level that switches and were aware of protocol adresses and gateways and so on. They were smarter.

A "switch" is a specific type of networking equipment. It is better than a hub, but not necessarily as good as a router. Wifi does not use or require it because of the method by which it works.

With (wired) ethernet, you can have switched because you can have a non-shared medium (individual cables not all hard-wired together like they would be in a hub). With wireless, you inherently have a shared medium...

The closest concept to a wireless switch is a signal booster or repeater, which are devices that do pretty much exactly what they sound like. Nobody calls them a switch though, because they're technically hubs (which are usually called repeaters when they only join two network segments), not switches. WiFi has a single shared medium, you can't selectively broadcast to only one 'port' because everything is on a single 'port', so it's a hub not a switch. It's pretty rare to see these devices outside of very specific circumstances though, as it's almost always better to extend the network using a wired connection and multiple wireless access points with the same SSID.

Note, however, that the term 'router' isn't always an accurate description of what most people call wireless routers. Most of them are capable of just acting a a bridge instead of a router, and it's not all that unusual to see them used in this way (for example, the guest network where I work is handled through a wireless AP configured as a bridge, with the actual routing being handled by our gateway systems).