Very late to the party but I thought the point "Why do you need an official campaign setting, why isn't a third-party one good enough?" is a good question and whilst Snarf and others have analyzed it well, I think there are a couple of things to add:
1) Artwork and design.
This is related to quality, but I think it's a bit different. People can call me whatever names they like, but I love settings and books which have a lot of artwork and good visual design, and my very long experience is that official books completely and utterly spank anything third-party as this. Sometimes there are exceptions, particularly where TSR/WotC decided to cheap out of just overemploy some eyeroll-inducing artist, but in general, WotC official stuff has been much better than third-party stuff, and it still is.
2) I hate knock-offs, and WotC doesn't licence out settings much.
So Planescape, for example, like would I have bought a third-party book if WotC licensed it out? Yeah maybe, but would I buy a knock-off of Planescape? No. I didn't even buy the official low-grade knock-off of Planescape, Ravnica (bored monkey ba-dump-tssh gif)! I don't want a knock-off because they always bloody change something they bloody shouldn't.
It's not really "authenticity" so much or "prestige" or whatever, it's like 80% of people who do knock-offs of settings just don't quite "get it" (it's interesting with "spiritual successor" indie videogames you see this a bit too - like, about 60% do "get it", but then 40% will just unnecessarily change something and ruin the formula). Like if you made 10 people make Planescape knock-offs, two of them would remove or ruin the Factions, two of them would kill the the plane-travelling thing, two would kill Sigil, and two would probably kill all three and just drill down on the Blood War or something. So maybe two people wouldn't screw it up, which is not good odds.
3) Depending on the edition, third-party may not even be viable.
I would strongly suggest that in 1E, 2E, and even 4E thanks to the godawful GSR, third-party settings were not viable. So this question, in a way, is really only about 3.XE and and 5E, which means it's a lot narrower than it initially seems.
4) Weird creepy stuff check.
Like, I'm not going to name names (not even in PMs, soz), but an awful lot of non-official settings for D&D and other games, like you're reading through them, "Oh yeah that's cool" and then suddenly "Uhhhh well that's incredibly creepy" or "Jesus that's messed-up" or "Wow what a horrifying change of tone". If it's like a horror setting that's horrifying, great, Scarred Lands gonna be Scarred and maybe scar you too, for example, cool, you know what you're getting into, but a number of other third-party settings it's been like "Oh boy...".
WotC are extremely unlikely to do that to me. TSR did have messed up stuff sometimes but I guess we can say the '80s and '90s lets them off a bit (albeit by the time Maztica and Red Steel happened it was too late in history to write either of those off as "just how it was back then" - both of those seemed messed-up at the time).