Obviously everyone is different. It sounds like, as others have mentioned, this particular freelance really values his freelancer profile. He is getting value from billable hours though there and is building his profile there even though he is giving up some of the money. His rational is, this site allows me to make a living, and they deserve their cut.
To me, personally, it's dumb. But I also understand their reasoning. The thing is developers are weird beasts that do things for many reason, and shockingly to some, it's really not all about the money. If you propose moving off of the platform and he says no, just go with it as there's nothing you can do to change his mind. He's probably too comfortable doing business though that platform for him to change for better or worse.
To me, personally, it's dumb. But I also understand their reasoning. The thing is developers are weird beasts that do things for many reason, and shockingly to some, it's really not all about the money. If you propose moving off of the platform and he says no, just go with it as there's nothing you can do to change his mind. He's probably too comfortable doing business though that platform for him to change for better or worse.