Pots are indeed owned by a single user, rather than being owned by an account. Which perhaps seems like the wrong decision in hindsight; I’m not sure. That’s why can’t just “turn them on” for joint accounts (or we already would have).
And it’s not as simple as changing pots to be owned by an account rather than a user, like we did for attachments. We’ve been doing work on new kinds of pots (such as a pot which pays interest), which has made the pots system a bit more complicated. But mainly there are some unresolved design questions: how should we display that you have two accounts, and a number of pots, and some of the pots are accessible to both of you and some aren’t?
I’ve seen some really exciting designs for reworking the Accounts tab to have a vertical list of all your accounts, rather than the current horizontal list of pots (which makes you swipe between them). I’m hoping that the relevant team will be able to look at this problem in Q4 (i.e. before the end of the year) 
Veering off-topic, but it’s worth pointing out that we think of “joint pots” (shared between two people who also have a joint account) as distinct from “shared pots” (a pot owned by many users). I would quite like joint pots as a way of putting our shared funds aside (currently we use savings accounts at another bank for this).
Shared pots are a whole different thing, and there are unresolved questions like who “owns” the money; is it still FSCS protected; and can one of your friends just run off with it all? Personally I think bill splitting, and/or a “long-running” bill split, would be a better solution. But as I said, I’m getting off-topic… 