Why is fixed better on wet?

Wet descent? I'll take the brakes over trying to slow down with my legs. Thanks.
Remember it's often not the braking force that's the issue, it's the tyre's grip. You don't want to be locking up brakes in the wet so not having awesome pad-rim grip is a good thing.

plus, the wet act almost like an ABS for bike, squeeze it hard and it won't bite hard until rotated a couple time, the difference is usually stopping an extra 2-4 metres than usual.

mind you in the wet, don't people slow down a little? a bit of a moot point, unless you're a raging menace hacking through traffic on oil-spilled road to knock on death's door.