There are a lot of ebbs and flows to why graded books come to the market
1). It was done before so there was already a business model. Graded coins and Graded cards.
2). Mail order suffered a high amount of returns.
3). Auction houses before Heritage sold most comics in huge lots. However they did pioneer "grading by committee" since most lots had 3-4 graders who graded it. Sotheby's lots were graded by Payette, Dan Greenhaugh of Showcase, Borock might have done some. I am not sure who else graded the lots. Auction houses did not allow returns so in those days you traveled before the auction and graded the lots yourself. Enter CGC graded books and all that preliminary what is the grade work goes away. Now it is replaced by the guys looking for upgrades.
4). Undisclosed restoration. All you have to look at is the census on some keys and see how many restored copies have been graded. Imagine that probably 75% and I may be low were being sold as unrestored. Honesty is not always the first adjective coming out of buyers mouths when describing the selling community.
5). I'll say again, no formal grading training. If a buyer is honest that they can't grade which I've heard more then once they are at the mercy of the guy in front of them selling the book. How else would they know to challenge the seller who they assume knows what he is doing? Which for me is laying a lot of trust into that sellers lap. And having traveled to a lot of shows and bought and graded a lot of books I'm not exactly sure that trust is deserved.
6). Books would not sell for a million dollars based on one sellers "opinion". The idea of impartial grading by committee has created an acceptance in the hobby that is rewarded by buyers stepping up and paying more for books then they would have before. Having GPA trackable sales has also helped give a stock market feel to the comic market. CGC registry sets have created the EGO driven need to have the best out there.