They don't stop development at E3. It looks like there is a form of mounts in the game, it just has a lot of bugs, and fixing those bugs takes time.
From my understanding, their reluctance to say mounts are in stem from them wanting it to be a feature that is beneficial, and not ridiculed for its implementation. Bugs may be an issue too, but it seems more like they just want to get it right.
Also, for Dragborn1's point, at a certain point there is going to be a time where they have to go this isn't working, and say they have to cut it and work on getting everything we know is gonna be in. Hopefully this doesn't happen, but the closer we get to release, the more likely it is.