advertisements need to have a lot of viewer in order to be potentially profitable for the content creators. The max revenue of an ad would be a few dollars per thousand views, however these rates are generally for video type ads that would be unlikely to be seen on a reddit thread.

Also people do not solely view a reddit thread because of the original content, they also view a thread because of the conversation occurring in the thread, and this is really not the content creator but the community as a whole.