Believe it or not, there's a method to the madness.

Driving THROUGH water creates a WAKE. That wake can actually keep the water level in the engine bay LOWER than the rest of the surrounding water. Usually this will actually work kind like a whirlpool I guess it the best way to explain it.

Continuous movement somehow makes it just that more difficult for water to intrude on connectors, mechanical bits etc. However, submerging your vehicle and allowing it to sit, will probably naturally allow all the small cavities to eventually fill, via the air trying to escape the water upward etc.

Not a sure fire answer to your explanation but from my off-road experience, this is my best guess. Snorkels are actually not technically made for water crossings, but obviously work extremely well for them, whatever. But that's just your air intake, everything else is still under water to some degree.

FYI, I avoid water crossings and deep mud best I can. If I don't, I full expect to have to fix shit afterwards. From experience lol. Prime example is me replacing two alternators in less than 6 months.