Were they waterproof? I certainly don’t remember my Nokia 3210 being even splash resistant.
> I have no idea why we can’t do this now
For better worse I think the answer is quite simple - a combo of ease of manufacturing, a long term trend of slimmer devices that is broadly supported by customer behaviour, and an extra place to suggest a device upgrade instead of a repair. You’re more than able to get pretty much any smartphone battery replaced with a manufacturer similar spec these days, and people still choose not to do so.
Waterproof phones with replaceable batteries existed, but they used more rubber and other tricks to pull it off. This had the advantage of making it a lot easier to close the phone again and still keep the water resistant properties.
Having replaced the battery in my old phone, I sure understand why people don't "choose" to do so. The battery itself is worth maybe 20 bucks but the labour and tools required to replace it cost much, much more.
Compared to spending 30 bucks on a secondary battery back in the day, things sure have gone downhill since the days of user-replaceable batteries.