The old Samsung Galaxy S5 was waterproof and had a user-replaceable battery, so your argument doesn't hold water.

Waterproofing isn't a yes/no feature. Modern phones are way more waterproof and dust resistant than the S5. The plastic back on the S5 would quickly develop cracks that would let water in.

Sure, it wasn't perfect, but it showed that it can be done, and it's not particularly difficult either. An improved design could have a metal back panel instead of plastic, it could use screws instead of plastic snaps, etc. (And with most people using cases, who cares about visible screws?)

If it's waterproof and you get the water on the inside, it should in fact hold water

It was really not waterproof. It was IP67, which is far from the IP68 almost all phones have today. https://phandroid.com/2014/04/21/galaxy-s5-ip67-meaning/

I thought waterproofing came late to iPhones, like version 7 or 8. What would the reason be before that given environmentalism as a strong motivator?

Getting a professional who will properly dispose of a battery is more environmentally conscious, and supports the local economy?