Okay, so if tabletop RPGs don't need to be sold with settings or campaigns, then why is this so commonly done? Well, as Lewis Pulsipher said in a comment, it stems from those turning their tabletop RPG into a business needing to find a way to bring in revenue to cover those costs. That's correct, and what Lewis says in that linked video should be heeded accordingly.
The practice arose in a pre-Internet era, when that was the only viable option for a business model. You sold those products on the argument of convenience, something that adult gamers know too well as being a thing they want, and you hoped that you satisfied their want enough to keep them bringing in more repeat business.
That's not going to cut it anymore. The Internet changed everything, including what makes for a viable business model for tabletop RPGs. It used to be that convenience, plus a common reference set to a central standard, made publishing supplements viable until that treadmill wound down. What you're really selling with settings and campaigns is information, first about an environment, and then about a specific course of action within it.
Guess what does that better than any book, boxed set, or pamphlet? A wiki. Don't tell me that's not viable; Infogalactic does that for everything and has better quality control than Wikipedia with a fraction of the funding and personnel, and they've already got a better Drudge-style news aggregator than Drudge does just added to it.
What's the best setting supplement to any Star Wars RPG? Wookiepedia. Yep, the fansite for all sorts of lore and such is better than anything published by Lucasfilm or any of its license holders when it comes to setting. If the volunteers can up their game just a bit in their article quality, they'll accidentally make better campaign stuff than anything so published. Even those who just read RPG stuff find wikis better for what they want nowadays.
But, since time seems to run weirdly in Tabletop RPG Land, this fact hasn't sunk in yet for a lot of people (even as they rely more and more on crowd-funding to keep their concerns going) despite me saying so to WOTC at a panel I sat with them about this very thing at CONvergence a few years ago.
The conditions changed. Your business model has to adapt, or your business dies.
You are NOT in the business of selling shit that you can get for free on a wiki. You are in the business of selling tools that your customers need to facilitate their engagement with the tabletop RPG medium. What you have to do to make selling the old way at all viable now is to castrate its potential as a TRPG product line, and in practice that means Organized Play (because centralized canon and cross-table rulesets are worthless otherwise; your brand, therefore, is your setting/campaign and NOT the ruleset) or you're going transmedia and need a brand for your fiction line (e.g. Forgotten Realms).
That's right, you're not selling D&D. You're selling Roll20. You're selling Tabletop Simulator. You're selling Fantasy Grounds You're selling the tools to make it easy and simple for players to get together and play the game AT ALL. The games--from rules on down--are not where the money is anymore; their value is only good for brand development, which has value when applied to other media (e.g. D&D in videogames, comics, novels, movies). Holding on to the model arising in the 1970s is a losing game, and the sooner you make the shift to where the real value is the better you will be.