They already considered it in the past, in fact Mark penned an email explaining why and how it would be beneficial to their broader ambitions to own more of the stack as a hedge for the future of computing. You can read more about it here:
https://sriramk.com/memos/zuck-unity.pdf
https://techcrunch.com/2019/02/13/facebook-mulled-multi-bill...
It will likely happen at some point, if not by Meta than it'll be another tech giant like Amazon, Google, Microsoft, or even Apple.