Because in order for people to build with that content, they'll have to buy it. But how many people will buy something to create content, with no one really willing to play it (upon release)? People aren't going to buy content expansions without having quality modules created with that content.
The only way to really work around this is to make the content "expansions" (for lack of a better word) would then be to make them cheap. The only thing is, creating content isn't cheap (it's arguably one of the most expensive part of game development now). In terms of finding ways to invest their money, a company would probably have a better ROI simply by making another game. At best, maybe they make a game with that toolset that's post-apocolyptic.
Since full-fledged expansions typically don't sell anywhere near the amount of copies as the original game does, I can't expect content (which doesn't really have any incentive for a non-content creator to buy) to sell any better. Except that I'd expect you to have to sell at a lower price, with less volume of sales.