Why is fixed better on wet?

That's more about technique though. On geared, if you're hitting a corner too hard, you brake but pull your weight over to the side whilst keeping the bike upright. I puts in a lot more drift but should bring you out just fine. Note: do not attempt this on a tandem, someone will die, probably you.

Sure, agreed. And a geared bike should be able to get through a corner quicker, but I think you feel more in control through the drivetrain on a fixed, and that braking can bring less predictable modulation (I'm thinking that's the right word), so brakes will grab at the wheel during the cornering. Not saying the wheel will lock, but the deceleration will be faster than controlling the speed through the drivetrain and more likely to step the rear wheel out. In the wet you're more likely to struggle to get it back in. Again, just my take on it from my experience