Lots of books just feels like a losing proposition.
Too much contents adds bloat, slowing down the game. It adds more rules to learn, more options to pick from when levelling up, and introduces power creep. Plus it introduces the wall of books phenomenon that makes getting into the game intimidating and introduces choice paralysis.
The counter argument of "then don't use it" just doesn't make sense.
First, because it requires a consensus not to use the content, which not all groups possess. If there isn't a consensus then there's conflict. Plus, people generally don't make wise decisions. Even knowing for a fact that certain content is unneeded or detrimental doesn't stop people. People do dumb things. It's human nature. ("I know this spicy bean burrito will give me heartburn and indigestion, but it's sooo tasty.")
Second, because WotC doesn't want books that people are encouraged not to buy or that the majority of players are not interested in purchasing. That's antithetical to running a business. Releasing more books means hiring more staff and means a larger portion of revenue from the D&D brand is expected to come from the RPG. The "optional accessories" business model didn't work particularly because you want people to buy the product.
The alternative that is bandied about is letting another company make the accessories.
Now, WotC / Hasbro isn't going to sell D&D. Ever. Only if Hasbro gets bought out by another, bigger company. Nothing has been done with MASK, Visionaries, Inhumanoids, or COPS in 30 years but they never sold those rights. Why would they do so for D&D?
Licensing doesn't work as well. This requires another company to give WotC money for the rights to publish D&D accessories. Licensing usually means less profit for the licensors but fewer expenses/risks. And it benefits smaller or more specialized companies who can also work with less profit. WizKids is a great example: WotC is capable of producing its own plastic miniatures and has done so in the past, but WizKids employees dedicated sculptures and software so there is reduced start-up costs, and WizKids has an established reputation and relationship with stores and distributors. Plus, WizKids is fairly large and can afford to pay the licensing costs knowing they can make their money back.
The catch is, licensees have to pay. And that's expensive and requires a lot of initial capital. Smaller publishers are going to be very, very hard pressed to pay licensing fees and then the production costs to make the books. It's not like you can Kickstarter getting a licence. Failure is also costly. Look at DungeonScape/ Morningstar. That cost Trapdoor Tech a LOT of money. Or the Marvel RPG from Margaret Weis Productions: critically acclaimed, award winning, and decently selling but still unable to move enough copies to make enough money with the licensing fees.
The thing is, there's *always* going to be too little content for some people. I remember listening to a podcast in the heyday of 4e (late 2009 or early 2010) when the magazines went from monthly compilations to weekly articles. So there was a new D&D book every month and new crunch every week and one of the hosts commented that "this means there was almost enough D&D content being released". Someone will always want more. WotC could release a hardcover book every other week and some collector would buy every one and wish to have something on the even weeks.
I love the Marvel movies. Even the weaker ones like Iron Man 2. I would go to the theater every two months. I want six Marvel movies every year! But I can understand that would impact the quality of the films and that most people will only go to see a couple each year. That the films do better if they're special and the more that are released the harder and hard it becomes to trump what has gone before.