They're waiting until enough work is put in to actually upset people, just as a warning to other potential criminals thinking of doing anything similar.

How have Nintendo’s lawyers not shut this down yet?

They’re never the quickest and usually only act if they learn of something from the media. That said, by asking for money via Patreon, the developer is also asking to get sued.

How come these people never learn?

“Never interrupt your enemy when he is making a mistake”

— Sun Tzu, The Art of War

In its current state this is pretty much a tech demo. Typically companies will use copyright if they see the end product as something that could compete against their current products while infinging their trademarks.