It’s pretty difficult to do that. iPhones are known to potentially lose some of their water resistance after a battery swap, as it’s hard to guarantee the replaced waterproof seal is as good as the factory one.
The EU battery regulation has exemptions for IP67-rated devices which retain 83% of original battery capacity after 500 charge cycles, which most modern smartphones will qualify for.
But we also want devices that are thin and lightweight. Watertight battery compartments are super easy (barely an inconvenience) if you "just" make the device thicker and heavier.
It is doable and with all the amazing achievements this is the thing that is too hard to get working? Also all new foldables are not water proof (yet).
Your MacBook isn't water proof either yet the battery is also permanently glued in. Why?
MacBook batteries have not been permanently glued in for quite some time. Replacement requires some disassembly but is perfectly doable for most techies.
Personally I prefer the current solution to adding weight and size.
I can’t remember my battery draining below 50% since I bought my M5 a while back. 10+ years ago I agree that the needs were different but these days needing swappable batteries seems like a very minor niche IMHO.