I suspect WotCs problem is that they simply can't put the time into iterative, complete-campaign playtesting. You'd need to run a bunch of groups through the entire campaign (faithfully as written!), then get all the feedback, synthesise it and make changes accordingly, then run through the same process again, multiple times, and then once you've finally got to a point where you're satisfied, publish the result as is without any further edits for space, word count etc. I don't know if this is how it's done. You probably have some groups testing the balance of individual combat encounters with parties of different composition, you might have some groups going all the way through but offering progressive feedback as they go, etc etc. And of course you probably have different authors working on the post-playtesting fixes to different bits of the campaign, and they might not be the same authors who wrote the material in the first place, and this whole process continues all the way up to the day you send the files to the printer. There's lots of places for things o go wrong.
Also to be fair, I suspect that if most 3pp campaigns were played by as many groups as WotC adventures, or were subject to the same level of scrutiny, then the same complaints would be levelled at them a lot more often. Writing campaigns is haaaard.