While I do agree that Reddit probably doesn't care, the ad endpoint approach is harder to implement than one would imagine.

Normally ads have strict requirements for the placement (show one ad every x amount of posts), which would be hard to enforce across all 3rd party clients. Not to mention that they might be targeted towards the user, measure exposure time, and record interactions. I think the only way to do this is having some sort of SDK, but they probably determined that the cost would be too high.