Here's the problem with trying to make a removable water resistant back for consumer devices.
These devices also need to be tough. However, to appeal to consumers, it can't be too bulky. So the rear covers tend to be more flimsy than those you'd find on dedicated rugged phones like a Kyocera.
The rubber gasket will do a good enough job against spills and a certain degree of immersion if the cover is perfectly sealed. However, rubber degrades over time and when the phone is dropped, there's a chance that the cover can either pop open or becomes slightly distorted.
It is possible, but manufacturers are aiming for a mainstream market, a market who's aiming for looks and durability. A rugged Kyocera is obviously not aimed towards the mainstream, so it can be seriously beefed up without issue. But on a mainstream Galaxy and whatnot, you have to make some sort of compromise.