It's not my area of expertise so I might just be delusional here, but from my understanding phones are difficult to make waterproof if they had replaceable batteries because they require a massive lid that spans across the entire phone while having a limited amount of thickness to work with and also because they have some expectations about the depth at which they remain waterproof. You don't necessarily have these constraints with a toothbrush. You can have a small opening at the bottom where you insert a tall battery and have plenty of height left to make a waterproof hatch. You're also unlikely to submerge your toothbrush in more than 50cm of water, like in the event of dropping it in the bathtub.
> Also, anecdotally, I’ve never had a sonicare toothbrush battery die. They still last weeks after many many years of use.
Sure, these exist too, though it doesn't reflect the majority of items which cheap out on all components, including batteries.
> Also, anecdotally, I’ve never had a sonicare toothbrush battery die. They still last weeks after many many years of use.
Sure, these exist too, though it doesn't reflect the majority of items which cheap out on all components, including batteries.