It makes me sad to see them claiming waterproofing as the reason, when the art of sealing easily disassembled components has already been perfected over many centuries... the other comment here mentions flashlights, another object of a similar shape which does often come in a waterproof version with replaceable batteries. I have no doubt they could design an electric toothbrush which is both waterproof and has replaceable batteries; they just don't want to, because they'd rather you buy a whole new one.

Adding to the irony, water ingress and resulting damage often occurs to them anyway, but not at the bottom where they seal in the batteries; instead, water gets in around the shaft seal and then corrodes the insides from the top. Once again, it would probably be an easy task to make a better seal design, but that would cost just a tiny bit more than they'd like.