It is purely the seals letting water in. You could use the best grease in the world, but it won't help much if water is present!
The seals on the engine / gearbox are stopping oil coming out you hit them with a spray of water they will let it in they are more resistant to water as there better designed and a different make up to bearing sealsIt is funny how seals on my motorcycle were able to stop the gearbox oil coming out or the water coming in at the exposed area of the chain cog. As I remember it was a seal only at the exit hole and not combined with a bearing. And all my cars spent years swishing through drenching rain without the wheel bearings getting rusted up. Why is it so hard to seal Bosch, Yamaha etc motors when hub motors seem to take mud and rain without such problems? Granted they are set higher up.
So do rusty bearings mind you lolAlso, seals add friction,
Guess they don't want any of that heavy grease adding weight to their bikes.top end race bike will run no or very little lube in them