Because both companies are worth many hundreds of billions of dollars? Meta couldn’t afford it.
I think people tend to forget that while there is a lot of money in tech, there’s a lot more money in money (finance) xD.
I don’t have better words to describe this but I believe, it is the lack of imagination. They already have the distribution channel, they should borrow an idea from Apple’s book and introduce something akin to Apple Pay. VISA and MasterterCard will be fighting over that partnership.