I have one public IP address. I have three machines behind it that I want to SSH into. How does the machine with the public address know where to route an incoming port 22 packet? For HTTPS this is easy - browsers send the desired site in the SNI field of the TLS handshake, so the frontend can look at that and route appropriately. For SSH there's no indication of which host the packet is intended for.
HTTPS and any other protocol that uses TLS has virtual hosting (because TLS has virtual hosting), and so does unencrypted HTTP (with the "Host" header), and some "small web" protocols such a Spartan and Scorpion. (In the case of Spartan, the domain name is the first thing the client sends to the server, which should make it easy to implement.) Like you mention, SSH does not. IRC and NNTP also do not have virtual hosting as far as I can tell, although I had suggested to add a HOST command to these protocols to implement virtual hosting.
Note that this is not the only meaning of "virtual hosting". It is very commonly used with different addresses or ports. For example, the Apache `<VirtualHost addr:port>` block. It gets confusing because this is the same block that was used for "named-based virtualhost" (different `ServerName` in the same `<VirtualHost>`). See https://en.wikipedia.org/wiki/Virtual_hosting
