to keep everyone busy all the time doesn’t work

I agree but that has nothing to do with scrum. It does not say how busy everyone has to be. Things like these are left for the team to decide. If you felt like you had to be more busy, then someone among you probably pushed it.

In fact i prefer a bit of slack time so people can react to incidents better. Imagine a 100% busy highway. Not nice. And then you need to spontaneously deliver something from A to B, using that highway -> there will be a traffic jam and everything works terribly. Have a bit of slack here and there so the spontaneous delivery does not mess up the flow -> much more effective and everyone is happier too.