It's no secret that implementing AI usually involves far more investment into training and teaching than actual code. You can know how a neural net or other ML model works. You can have all the code before you. It's still a huge job (and investment) to do anything practical with that. If Meta shares the code their AI runs on with you, you're not going to be able to do much with it unless you make the same investment in gathering data and teaching to train that AI. That would probably require data Meta won't share. You'd effectively need your own Facebook.

If everyone open sources their AI code, Meta can snatch the bits that help them without much fear of helping their direct competitors.