I did notice this & I have found quiet a few nice stamps on postcards and postmarks to. They seem to sell very cheap to. Its maybe possible to find scarcer stamps on postcards so I'd say theres no limits collecting them. You can get Zeppelin postcards which are nicer than some Zeppelin covers as an example.
In general, postcards are a tough way to find scarcer stamps because they were carried at the cheapest rate, and there were very few opportunities for uprating ... perhaps the most common opportunity was domestic postal cards being uprated for airmail, back in the day ... though here on SCF we saw a post(al)card sent registered mail, and it could not have been the only one.
What is easier to find are stamps that were rarely used on a postcard, but that's not gonna excite everyone.
Slogan killers & unusual town names are more easily pursued, and I enjoy both.
Still although at the cheaper rate I have seen nice Latin America stamps and also Canal Zone among others on postcard. Also postage due stamps. You also get a nice image on the other side but your right it won't excite everyone. Here is a nice one I got from Spain