We know about yours

And the intrinsic problems. You also forgot to mention the software issue on the 2.1's which caused micro pulses which put additional strain on the sprag/belt - hence the warranty extension on those bikes (and yet still the flat earthers refuse to upgrade their firmware and will then complain when their motor fails).
Yes, water is a motor killer. Get that inside and things start to rust up and kaboom - your motor's shot. Better sealing and stronger sprag bearings would be the dream, and fortunately for us Bearing man makes the dream a reality. Maybe you could argue the motor should be like that as standard, but you could still upgrade your motor with Bearing man without waiting for a fail if you wanted to. Just as some people will extend the breathers and fit a snorkel to their vehicle rather than drowning their engine.
There's a whole host of reasons why. Some people are just unlucky. Some people jet wash and don't care. Some people ride through deep water and pretend it never happened.
The ones that are interesting are the 3/4/5 motor people. The majority of people have no failures, yet some have multiple.
Is this due to the dynamics of how they ride which is somehow incompatible with the way the sprag works ?
Do they have more pedal strikes, or some other form of impact, either because of where or how they ride ?
Is it how they transport their bikes ?
Is it something to do with how they wash it ? Is the motor warm and the air in the motor warm so as it cools with washing, the air contracts creating negative pressure and actually drawing water in ?
Is the bike so loved, it's washed in the bath ?
I'm not saying any of these are what happens, or that any of these people purposefully abuse their bikes or are negligent, but there must be some pattern which is different for multiple motor failure people than for others. It would just be interesting to try to work out what that might be.
If you knew, you could try to avoid what causes the problem. I doubt anyone wants a failed motor.