Agreed, dynamically serving features based on a database flag is just one part. There's A/B analytics, rolling out to specific user segments, recognizing stale flags etc...
You could always build it all your self, but I guess you could build a lot of things your self...
Beyond that, who's maintaining it? Are you going to be able to deploy these things across every single one of your systems? Your website? Your apps? In conjunction?
I absolutely understand that you could just read somejson from a cdn and call it a day, but at some point someone is going to have to maintain it and become the defacto feature flag person lol
You could always build it all your self, but I guess you could build a lot of things your self...