Real money gambling is legal in many non-US countries. It's also legal in many individual US states.
https://en.wikipedia.org/wiki/Gambling_in_the_United_States#Legality
The problem is that if they make real money betting available to even just one person somewhere in the world, everyone else can sell their mana to that person and it becomes effectivly real-money everywhere.
Maybe that'd be ok as a legal fiction, but probably not. (If it were possible I'd expect e.g. Polymarket to have done it.) But what if they banned the sale of mana from people in areas where it can't be cashed out to people in areas where it can? Or created a separate currency that can influence the probability of the same markets? There might be some way to make it work.
Market resolves subjectivly once I'm reasonably convinced of the answer. I'm making this as a discussion post and a way to gauge a general "vibe" of whether it's doable, rather than as a rigerous prediction. I won't bet.
Perhaps a little off-topic, but even if it were possible I don't think it would be desirable (for manifold specifically). A lot of what makes this place fun is that ~everyone on here is in it for the love of the game. If people were trading to pay rent I don't think it would be the same thing at all.
There absolutely should be real-money prediction markets though. Ideally ones legal enough and with enough volume that institutional investors would get involved.