Before #PrintOnDemand#POD and online book sales, a book run was generally 10K copies or more. Someone had to organize & oversee the logistics of moving them all to stores & maintaining stock at the point of sale. Plus, 10K copies requires a lot more storage space than you would think. That's also another cost.
If you were lucky & your book sold more than 10K, another print run were another cost. Translations & international sales meant more expenses.
Back then, the only form of real "self-publish" was #VantiyPress, which was as much of a ripoff and waste of money then as it is today.
Basically, it's easy to be comfortable in this modern era and think that things have always been as they are now, that the #arts were always something that #artists could do on their own and pay the rent. It's easy for younger people to think that the middlemen were always unnecessary, but that's not true.
rysiek