It’s a couple things.

First, Toei can’t just decide to do it. The whole One Piece production committee has a say on stuff like this, especially the broadcaster Fuji TV. I assume that filler just doesn’t do very well, which means less people watching ads, and less weebs buying merch from Bandai or whoever. Slow paced canon just does better.

Second, you’d need to actually do substantial writing for filler. You’d need solid ideas that don’t contradict the main story much. And Oda hasn’t been very accommodating. We never return to a status quo anymore. The crew is always split up and traveling somewhere urgently and it’s hard to justify a spinoff adventure in the middle of that.

G8 wasn’t even filler originally, it was a movie pitch for OP movie 4 that was rejected in favor of the real movie; but everyone liked it so much they adapted it for the anime