The usual argument manufacturers present is that water-proofing a phone involves having its interior be as completely sealed as possible, whereas a removable battery obviously requires that its interior be at least vaguely accessible, so it makes water-proofing substantially more challenging. Additionally, they can’t be as efficient with packing the internals tightly since the battery has to be accessible without completely disassembling the entire phone, so devices have to be a bit thicker.

I won’t pretend to have enough knowledge about device manufacturing to known just how sound those arguments are, but that’s what they say.