There needs to be political will too. There are huge risks involved in taking on monumental projects. What if NASA spent billions of dollars developing a major project that fizzled? If there's no political power behind it, NASA risks losing all of the funding they get in the first place, and definitely not being able to get more to finish what they started. As an organization, it is safer to survive doing lots of small, achievable things, and wait for the day that the world-changing missions are an important political tool again.