Well, by my educated reasoning, it's the following:
One: Back in Brawl, they were introduced as a way to have characters get involved in battle without needing to be playable.
Two: They typically only get promoted in the following game, if any of them do. Not even DLC changes this.
Three: This is one of the few habits the staff doesn't seem to deviate from, probably since it'd be preferable to divert effort to surprise newcomers while still having ATs for variety's sake on the battlefield. Well, that and possible programming issues (
's blocks being put on every stage resulted in severe glitches, so you could imagine how that'd be like for accommodating an AT).
Four: Some of them are openly acknowledged in a joking manner, which seemingly points towards them not being intended to be playable for now, because it's rare for Sak to explicitly bring up anyone that's minor content if they were intended to be a surprise later (hello
).