That's pretty much how it used to work.

The main reasons why it changed are:

• Space, as a connector is slightly bigger than a soldered connection, and the battery itself has to be slightly bigger and stronger too to be safe to transport and handle without being protected by the phone's case.

• Water resistance; it's far easier to make a phone's case waterproof by just glueing the whole thing shut than having to use seals and gaskets and such to make it possible to open and close it at will.