Just answering with a possibility here, but they could be seeking freedom from liability for failure to moderate content or ensuring their service is "not harmful". If it's only for consenting adults, and every adult can be pinned down with an identity, whatever happens can have the blame assigned away from meta.

edit: I took too long to write this :)

And they can offer an age verification product now too.

I wonder if they plan to provide "services" to web sites that need age verification... via their app and Facebook account..

As someone that doesn't have a Meta account (and will not), this could become potentially problematic.