Bedrockgames
I post in the voice of Christopher Walken
TSR tried to crank out book after book to mostly DM's who were a smaller percentage of the customer base. And then they narrowed it down to only certain types of fantasy and play a smaller subgroup loved. So TSR was trying to make all their money off maybe 10% of the consumer base.
The books were not exclusively for the GM. The complete books for example, all had material players wanted to. Also while the game books were written with GMs in mind, players regularly bought them (believe me I had plenty of players who had all the Ravenloft stuff I was running, it was very common). Also people really underestimate the impact of aiming these books at the GM. In order to have a thriving game, someone has to be the GM. It is the hardest job in the group (when I don't feel like GMing, I choose to be a player because there is simply less responsibility and work). Books that give the GM guidance, ideas, inspiration, adventure material, etc. These are all gold for keeping D&D campaigns healthy because they help maintain GM interest. I liked 3E, but my experience with that as a GM was it felt enervating. It was enervating for a lot of reasons but one of the main ones is there wasn't a whole lot out there that was good, the way there was in the 80s and 90s, to give the GM a hand, to give the GM inspiration. And the stuff that was being put out, just created more of a headache for the GM because it was player splat material.