Like all successful businesses, they need to analyze ROI. Which is why it's a good thing they support 3PP, and seem to be actively looking to add more support with DDB. Let those other companies fill in the blanks, take the risks and provide options for people that want them.
Meanwhile if most people don't use extensive 3PP (and in my experience, most do not), it's likely because they're happy with the core game and the options it provides. Because for some people, no matter how many options they add it will never be enough.