Because any product that WotC markets is going to sell more than an identical 3pp because it has the imprimatur of being "official", which is a very important consideration to a wide swath of the playerbase.
If you think WotC material sells hundreds of times the number of copies of a book than a 3pp because their material is hundreds of times better, then we have very different views of the marketplace.
It has nothing to do with second guessing WotC. I don't particularly care what WotC sells. But what sells for WotC is the mark of officialness and widespread acceptance, not the quality of the material per se. Now, if WotC starts to release a bunch of shoddy, substandard material, that can certainly erode the value of their imprint, but we're very far from that point right now. 5e is quality material, and also at the right place at the right time, which is why their current material is so very successful. Much like each individual MCU movie, being part of the greater whole enhances each individual product.