Animations take a while to create and there's production time involved in that. Selling them on the Mog Station offsets that cost. We still do get new free emotes from time to time, but generally within the patch cycle as part of other content where they needed to create that animation anyway. The same thing kind of goes for voices where they need to pay someone to create the voices and incorporate them into the game. They likely don't see the return on voices the way they do on emotes since it's unlikely they will ever cash shop voice packs... But you could always ask about it in a Q&A Live Letter and Yoshi might consider it as he's done with other things.