Re: Why does nobody build disk libraries?
They do make them; are your referring to something like this?
(first hit from google "optical jukebox" that wasn't a wikipedia link)
TL;DR - Yes, but they have the same problems. They are a bit faster for access times.
Optical jukeboxes tend to have two problems:
1) Relatively low data density compared to tape. (a BD-RW gets you what? 50 GB tops per disc?) While the discs can be crammed into a smaller volume, You need more of them to equal tape. One unit is 5 feet by 15 inched by 28 inches and only manages ~70 TB max capacity. An equally sized tape library will beat that without breathing hard. (oddly enough, they cost the same...)
2) They suffer the same problems as tape changers. In the ~8 years that I've been overseeing backups at my current employer, we've gone through four tape libraries; the first one was died from old age and 'we don't support that model anymore'-itis, second had the robotics die on it requiring a chassis replacement with unit #3, which was superseded by the fourth, which has had the controller and most of the drives on it replaced in it's 5 years of operating life. They both have a good deal of high precision moving parts which are fiddly, finicky, and sometimes downright cranky.
They do have some pluses, though: faster access time than tape, and depending on the media fed into them, better archival longevity. They also have random access capability as well, so you don't have to de-spool 3/4 of the tape to get the single 100 MB file you are after near the end of the media.
My first corporate job was with a credit union that had two optical libraries: a moderate sized one for mortgage records (about the size of a half-cabinet), and the second was this giant beast of a unit for check image storage. (it was larger than a double wardrobe) and that was in the late 90's, so that was CD-R or M-O media.