They've been reducing ingress points over time. The removal of the headphone jack was at least partially for this reason; so was the non-mechanical home button.
A battery door is a giant potential ingress point for water.
No, those are just more ways to stop people from opening their phones and doing things like replacing batteries rather than buying a new phone. There are water resistant phones with headphone jacks, removable batteries etc. Shoot even a plug for potential infress points would be fine. Sorry, I just hate the new anti-consumer trend of irreplaceable batteries. Feels like we are back to the days where every phone had a diff charger port. Anything to make people spend more money than they have to. /rant.