Because of complexity perhaps? Managing HTTPS certificates, and getting them to work in the first place, is hard enough. I can't imagine how much more difficult manually creating and adding certificates to their nodes or other arbitrary software would be, unless bitcoin-seeder gets the functionality to do those things automatically merged, which is unlikely.
It's not the nodes it's the DNS for the nodes.
https://en.wikipedia.org/wiki/Domain_Name_System_Security_ExtensionsJust about every major DNS provider supports it.
If you want to host a seed node it just seems like a good piece of extra security to be sure that when someone looks up your address it really is you on the other side.
-Dave