Windows 10 Action Center doesn't show notification history

I'm asking about why the notification "toasts" (again, I don't know the Windows terminology) are never making it into the Action Center in the first place.

The toast notifications are transient by definition, so they vanish after a bit of time, unless they require action.

From MS:

"A toast notification is a transient message to the user that contains relevant, time-sensitive information and provides quick access to related content in an app."

It's up to the app's developers to pick the notification type. Since the Action Center is an extension of the toast notification system that existed in Windows previously (since like XP), notifications triggered using old APIs/methods will act like they used to, which includes not being added to the Action Center list.

You can control how long transient toast notifications stay visible via Settings -> Ease of Access -> Other options -> Show notifications for, where you can set them to last onscreen for up to 5 minutes.