> I have no idea why we can't do this now.

Power density and all of the problems that are fallout from high power density. A modern LiPoly battery contains a lot of energy. It's stored in a chemical that is highly reactive with oxygen, which you happen to be surrounded by at the moment.

Watch some videos on lithium battery fires. Not only do they have extremely hot flames but they spray out molten bits of battery and can't be extinguished with water.

To make a LiPoly battery that can be safely handled by normal people it needs a strong outer casing. A sealed battery in a phone uses the phone's case itself as the battery's protective casing. A user replaceable battery would need its own strong case. The phone then needs to be larger to accommodate the extra volume or have a smaller battery which stores less power than a sealed battery of equivalent volume.

The market has rewarded thinner phones with long lifetimes but also high performance. That triple point is challenging if not impossible to hit without sealed batteries.