Thanks for the reply, Erin. There are, unfortunately, a couple of fundamental issues that I'm struggling with.
First, this doesn't really answer the question why this happens. I have not synced my phone to iTunes in a VERY long time, so why do these songs disappear? For example, I vividly recall listening to "Grey Street" by Dave Matthews Band in my car driving home from work in the spring. I tried to locate that same song just a couple of days ago and it is nowhere to be found. I am 100% certain I have not tried to change anything with my music, including syncing it with iTunes. So, where did it go?
Secondly, why is the onus put on the user to "import them again" when that work (which is relatively painstaking at the time) was already done? Some of those CDs have been tossed or long forgotten. Again, an example, I remember ripping Weezer's blue CD because of all of the wear and tear on the disc itself. This was a way to preserve those songs while not depending on the CD to scratch its way through my car's CD player. Once it was ripped onto my computer and made it into my music, I discarded the CD because it worked well where I had it. Now that entire album is gone from my music.
Third, and lastly, this does not account for those files that were hidden gems before iTunes was in existence. I'm certain I lost songs that were downloaded by Napster (before the litigation) and other programs that existed when music downloading was like the Wild West. There are at least 20 Pearl Jam songs/Eddie outtakes that are missing that I know I had a year ago. There is no way to retrieve those files again. Did iTunes do some policing of files that were not obtained through their platform? A majority of my missing music stems from this space.
The reality is, of course, I'd love to recover all of this music and these files. But, I also know, that is unlikely to happen. What I am more interested in at this point, is trying to understand why my music files would magically disappear (and disproportionately the files I did not purchase through iTunes)? This is not intended to sound like a conspiracy theory, but is Apple weeding out music that is not compatible with its library? In other words, they don't have it on its humungous server, so you can't have it on an Apple device? I have no problem with that being the answer. It would just be nice to know so I could prep myself for files that are important to me to be removed without my consent.