why is the caravel wrapper required (as opposed to optional)? is it because the fab needs to be able to test the chips, and they couldn't do that with a fully custom design? or is it because there are packaging details that just can't be made public enough for you to design your own?

As far as I know is because your design is inside of other design, this is call the harness, and that simplifies the design of you asic, making available the I/O, that is not a simple wire, and the Risc V core that manage your design. Creating this wrapper allows efabless to use that standard harness to the standard interface that is the wrapper.

Its not a fab requirement and is to create a standard interface for packaging for all MPWs. It also provides convenience for the designer.