While yes, not every book is for every person, you need people paying attention to your announcements for them to know which book they are interested in. If I decided to skip the January, February, March, April and May releases... am I even paying enough attention to know that June is a book I'd really love to have?
Additionally, there is an inevitable dip in quality, because the team would need to do three times as much work per year. You could maybe get that quality back by hiring more people, but considering WoTC just cut a bunch of people, that would be a hard sell, and if the products don't all sell at the same rate as the current books do, then each book needs to be more expensive to make back the additional money spent on the larger team... which makes people less likely to buy, especially since people say the current books are too expensive.
I agree, it could theoretically be done profitably, but after training the player base and the employees for the last decade on the current release schedule? I don't think it could be done in a practical sense.
Well two things.
1) You also need to account for how many books a person would need to carry with them. Traveling GMs are a thing, and the more books you need to carry, the less you will invest.
2) Again, especially for the Humanoids, Keith Baker has already released that book for 5e. I don't remember if he covered the orcs specifically, but the Ghaal'Dur were covered in quite a lot of detail. Now, this isn't true for all settings, but WoTC does have to weight the potential benefit of an "official" release against the fact that a highly respected creator is using DMsGuild (which makes them money) to sell the same product. Would they make enough money to justify the cost, especially in the face of an existing revenue stream for the same content?
I just don't think they would.