In case you didn't know, domain names form a tree. You have the root ., you have TLDs com., and then usually the customer's domain google.com., then subdomains www.google.com.. Each level of dots typically hands over the rest of the lookup to another server. So in this example, the root servers tell you go ask .com at this IP, you go ask .com where Google is, and it tells you the IP of Google's DNS server, then you query Google's DNS server directly. Any subdomain under Google only involves Google, the public DNS infrastructure isn't involved at that point, significantly reducing load. Your ISP only needs to resolve Google once, then it knows how to get *.google.com directly from Google.

You're not just buying a name that by convention ends with a TLD. You're buying a spot in that chain of names, the tree that is used to eventually go query your server and everything under it. The fee to get the domain contributes to the cost of running the TLD.