[Edit: Never mind, others have explained elsewhere in the discussion. It's the lawsuits Facebook is losing for addicting kids. So rather than, you know, stopping doing that, they want to instead legally force us to alter every OS on the planet. Disgusting.]
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.