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.
These phones weren't very popular in the west (Japan in particular had a large waterproof phone market, allegedly because people were taking them with them into the shower), but they have existed for a years, before the first Androids or iPhones became water resistant.
Phones like the Samsung XCover (the Android 2.3 one from 2011) were sold worldwide and featured a hotswappable battery. Over time these became bulkier and frankly uglier as they doubled down on how "rugged" they were, but the technology was clearly there.
ip68 rated from 2020!
If you go further back, you have the whole "active" series of samsungs, eg. galaxy s5 active (ip67): https://www.ifixit.com/Guide/Samsung+Galaxy+S5+Active+Batter...
Samsung actually made such phones even in 2020, eg the xcover pro: https://www.samsung.com/uk/support/mobile-devices/how-to-rep... , the phone is rated for ip68, so yes, waterproof by phone standards.
> 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.