Why is fixed better on wet?

Thinking about it for me, with brakes you have to scrub speed before you turn in the wet. If you misjudge the speed and are too hot you then have to scrub more while leaned in you run the risk of the rear wheel breaking away.

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.

does make you wonder why roadies don't ride huge heavy badly made bikes, surely they'd get even more exercise?

There's exercise and then there's bad exercise. I do know a few roadies who go for a spring tour before the race season and swear that hauling an 80lb bike over long distances helps them. I know I'll rarely be as fit as I was after cycling across Canada.