I never used CircleCI so I can't speak to it specifically, but there's tons to improve on GitHub actions.
The Actions web UI is so bad it's almost a parody. Debugging stuff is harder than it needs to be. No good way to manually control builds. Possible but not very easy to run stuff locally. Very limited platform support (Linux Windows, and macOS, although other platforms are possible with VirtualBox hackery but it's SLOW and pretty unreliable).
It is enough to make a viable business when GitHub actions is right there? Who knows... But there are tons of reasons for external CI tools to exist, IMO.
The Actions web UI is so bad it's almost a parody. Debugging stuff is harder than it needs to be. No good way to manually control builds. Possible but not very easy to run stuff locally. Very limited platform support (Linux Windows, and macOS, although other platforms are possible with VirtualBox hackery but it's SLOW and pretty unreliable).
It is enough to make a viable business when GitHub actions is right there? Who knows... But there are tons of reasons for external CI tools to exist, IMO.