Ejecting a Disc

The most common reason is that the disk has open files.

Some Application you are still running may still have files listed as open. You may not think those Applications are still running.

In recent versions of MacOS, Application behavior has been changed. Now when you close all the Windows of an Application, it goes dormant in the background, and does not Quit unless memory becomes really tight. This is done as a conveniences for you, so that you could re-launch that Application instantly.

If you are not using an Application, and do not care if it takes a little longer to re-launch later, be sure to choose Quit off its File menu, rather than relying on closing its last window. Battery life may also improve, but likely not by a measurable amount.

If this is driving you crazy, there is a way to determine exactly which files are in use by each Application. But there are really a LOT of files, and this process takes some work.

Select an Application in Activity Monitor, and click the [ (i)] Icon for Info. In the window that opens, choose the Open Files & Ports tab.It will show a scrolling box of all open files. Stretch that window to see more. The number of files it has open is often shockingly high. A simple textedit Application may be supported by 30 or more open files plus eight or open ports.

You asked "Why?" so I gave the long answer.

If you just want to eject the disk, a Restart will free it up, unless an Application re-opens files on it.

Or do a Shutdown and disconnect the drive, then start up again.