Since you asked nicely 
here’s my attempt at answering that question -
TL;DR there’s lots of legal complexity.
Also, Monzo have talked about shared Pots being used for a variety of use cases, not just your traditional joint accounts e.g. by flatmates pooling money for bills which could make them more difficult to set up.
The caveat there is Monzo tends to come out with an MVP that covers 80% of the use case & then build on it so that might not be a huge blocker..time will tell!