What is your assumption based on?A user replaceable battery is only an engineering solution away. It can be done whilst maintaining IP68 integrity, it’s just manufacturers don’t want to because they’d rather customers buy a whole new device and contribute to the ewaste mountain.
Something simple like a bottom plate that unscrews allowing the battery to slide out wouldn’t be too hard.
What happens when people strip or lose the screws?
How can you maintain waterproofing with removable covers?