Its way easier for the speaker ports, you just seal the drivers well enough and they'll be sealed unless there's just too much pressure. The seal never experiences any mechanical wear, you can practically just glue it around the edge.
Similar thing with the charger/data port. The outside of the port can be completely sealed up with just the electrical connections going through. The port isn't ever opened, there's no mechanical wear of actually going in and out of the sealed area. Glue it all up but leave the electrical connectors exposed and its sealed.
A battery door is a whole 'nother issue. Starting off, its probably going to have considerably more perimeter needing to seal, especially if its like the doors of yore where you popped off a significant part of the back. Then, you'll need this seal to handle a lot of open/close events and be able to handle the dirt and debris which it will be exposed to. Keeping the device's profile thin gets way more complicated with all of these requirements, and the seal will probably still be less reliable than the seals for the charging port and the speakers/mics.
Similar thing with the charger/data port. The outside of the port can be completely sealed up with just the electrical connections going through. The port isn't ever opened, there's no mechanical wear of actually going in and out of the sealed area. Glue it all up but leave the electrical connectors exposed and its sealed.
A battery door is a whole 'nother issue. Starting off, its probably going to have considerably more perimeter needing to seal, especially if its like the doors of yore where you popped off a significant part of the back. Then, you'll need this seal to handle a lot of open/close events and be able to handle the dirt and debris which it will be exposed to. Keeping the device's profile thin gets way more complicated with all of these requirements, and the seal will probably still be less reliable than the seals for the charging port and the speakers/mics.