I guess it's just better to believe that there was actually another species that actually made them for us but the ego of Cooper is so big that he believed that it was themselves from the future.
No, he reasons that it must be humans because why would a completely alien, 5th dimensional race give a damn about some primitive lifeforms on the 3rd planet of a minor star in a minor galaxy.
Post-humans are conjectured by Cooper to be a 5th dimensional race. (Post-)Humans would be very interested in setting up the events in the past(woah slippery slope) that set the stage for the eventual emergence of post-humans.