From Wikipedia: "Jim Davis, the creator of Garfield, approved of the project, and an official Garfield book (also called Garfield Minus Garfield) was published by his company. It was mainly edited comics by Walsh, with some comics contributed by Davis."
[1] Garfield was originally created by Davis with the intention to come up with a 'good, marketable character' https://en.wikipedia.org/wiki/Garfield
Newspaper comic artists aren't working for free. They all want money. That's why they work.
My family were huge Garfield fans growing up and had a bunch of the books (one in German). The side characters were fun Odie, Lyman, the overly adorable kitten (Nermal), some relatives that came from a farm or something.
The “worst” thing was at some point it did seem like Davis was cranking them out for the newspaper without some of the care (though it might be I overdosed and became kind of sick of them). The other characters disappeared or became infrequent.
I don’t begrudge him though.
I think he kept his slot more out of nostalgia and risk adversion from the papers than anything else.