Well, you have a point. But only 5 out of 19 are worm on top, and all those are AC with big motors.
It looks to me like worm on bottom is the preferred mount except when the motor is so big its not practical.
Since that is where 99% of the load, wear, and heat is generated it makes sense to have it submerged in oil.
Plus, every worm drive winch on trucks, boats, wreckers, etc I have ever seen had it on the bottom.
However, since most gearboxes have the full plug at the centerline, along with a fill/drain plug top and bottom, I suppose it will work either way.
But, of the 5 worm-drive winches in my yard right now, all of them have the worm on the bottom......