It's super dumb. I assume it was really hard to get the whole world on board with the WTO to begin with, and trying to get them to join a WTO-except-USA wouldn't be any easier, especially with the USA trying to shoot it down.

A bunch of those countries will have signed national laws and international treaties deferring to the WTO which can't just be updated without renegotiating. And changing the WTO itself would require simultaneous unanimous agreement from all countries besides the USA, and they'll all try to get their pork barrel in.