So Time Machine doesn't backup the contents of your Books library

Just turned off iCloud sync on my iPad for Books and was planning on manually adding them from my iMac.

Go to my iMac and everything I put in my Books library for probably years; pdfs, epubs, etc. it's all gone. Why would books I added to my friggin' iMac not still be on my friggin' iMac? I get turning off iCloud sync deleting them off my iPad but one would assume documents I added on my friggin' iMac would still be there.

And then to find out that Time Machine didn't back them up either?