I consider Belcher to be a serious threat, but nobody plays it.

The other 4-0 deck at the Pro Tour side event was a Belcher player. I squeaked by with a bit of luck and he got eliminated by Enchantress of all things to finish in 5th or 6th place (extra round + cut to top 4, so he was out). I consistently see a Belcher or two hanging around the top tables. You can't confuse the lack of a tournament win (or the lack of extremely high-density high placement finishes) with the deck being bad. There just aren't very many people playing Belcher. I think just that guy in the PT and in the other two SCGs events (the ones we have data for), there were only like 6 people playing it and 4 of them finished 17th place or better.

I've never seen a Belcher player win less than half of its matches, and many that put up very impressive finishes.

You invited the ANT comparison, so I'll point this out: ANT has a few tournament wins, but also tons and tons of people play the deck. Saito and Do Anne are just the tip of the iceberg, you don't see the 50 or 100 ANT players who finished out of the money, out of day 2, and even with a sub-50% win ratio.