Making Saved / Imported Files in a Multitrack Project "Stick"

I have been toying with switching to Audition from another DAW, but I have a general workflow question for those of you who use it regularly, since I find some of it's behaviors extremely non-typical.

For example, why are imported files not saved inside a multitrack project unless they are actively in the project editor?

Say I have some audio files on my HD pulled from an SD card, or another DAW where they were recorded. I open Audition to mix and edit, so I create a new multitrack session and save it on my desktop. Now I create a folder called "Audio" inside the session folder, drag the files I want to work with into it, and then import them using File -> Import.

So, thinking my files are "imported" and my session is all set to go, I'm gonna grab a snack. I save and close.

Then, next time I reopen that session, nothing: no files in the file view. So I drag one of the files into the editor. Then I save and close. When I reopen, that file shows in the file viewer, and the rest are gone. Hmm.

It seems crazy to me, especially with "multitrack" work, that you would have every single file in the editor at every single save point in the process of building a track. Say I have a horn hit that I know only comes in at the end? Or say I'm doing audio for a podcast that has a pre-recorded post-roll? It seems like those files would need to be re-imported every single time I save/close the file, unless I just dump everything into the editor. But that's the kind of chaos that I'm trying to avoid. Is there a setting to change this behavior?

Thanks for any feedback or help.