I think the answer is that public WiFi hotspots don't have any way of verifying that they are who they say they are, whereas TLS works because you have a truste

I think the answer is that public WiFi hotspots don't have any way of verifying that they are who they say they are, whereas TLS works because you have a trusted third party (the CA) that verifies the keypair. So, if a WiFi hotspot used something like TLS, the data would be encrypted, but you have to way to verify that you're not going through a malicious third party's hotspot on the way to the public WiFi.