I'd say partly due to SCSI always been aimed at the corporate end of the market, partly due to SCSI being obsolete for awhile now so no longer new units being manufactured and partly due to being somewhat rare. I say rare as I've seen 100's of servers in my time, over 50% of them having SCSI hard drives but always IDE CD drives (if any)
The only SCSI CD drives I've seen in the wild were in a Compaq CD tower.

Not only on ebay, but i have seen people buy SCSI CD-ROMs for rigged prices of 50 euro and even more.
Some write silly stuff about certain brands like Plextor having god like quality of burn for Audio and sell like that.

All SCSI hardware i ever bought was below 1 euro. Even had one guy over for some talk, ended up giving him an Plextor 4x caddy CD burner for free, he was as if he received 10k $ 🤣

SCSI is for the big boys!

Definitely 😁
Thats why my Vectra is SCSI only. Aww yeee

Ha, good thing I stocked up when the pickings were good. I have a Plextor 32CSi, the fastest SCSI CD-ROM drive to use a caddy (as far as I am aware).