I think to some extent it's the classic error made by people make who don't understand marketing; the concept of the Unique Selling Proposition (USP). People who use price, discounts, or free stuff as their USP find that there's nothing unique about it, as their competitors quickly match whatever price/discount is offered, so a USP that consists of price (or discount) is the worst possible choice.

But... since there are so many webmasters with no business sense, and so much competition these days, many webmasters feel forced to compete and develop a USP in whatever way they can. Adding more content or giving more away is the easiest way to do this.

It's a lot harder to develop something really unique that no one else can offer and thereby take one's site out of the competitive marketplace and sell it based on whatever USP you develop. It can be done, but I don't think anybody thinks about it.