IMO: The reason it hasn't happened is because there hasn't been a reason to do it.
The current size of all AP servers, combined (that aren't owned by friggin _meta_), is basically trivial and it's an excessive amount of work and numerous legal headaches to integrate even just with mastodon.
To EEE you first have to ermbrace it, and there just isn't a reason to do the first E for most companies.