I think part of the reason is just to keep communities thriving. Smash reveals have become these huge events that everyone loves and predicting who among the entire universe of gaming could be the next representative appears to be a favorite pastime among Smash fans, so I think keeping them secret for that reason alone makes it worth it.There's like zero reasons to keep them secret though.
It's like they're trying to use them to just fill time since they have nothing to announce.
I could have done with a teaser for the next fighter though. But waiting does make it kinda fun tbh.
Then again, December of 2021 is when they promise to have everyone out, so maybe the "fill time" thing might have some credence to it. Sakurai did say that they already decided the characters, so I don't know.