It is just hard for people to differentiate between adding/changing significant game code and creating tools/tweaking game code. There is a huge difference. It's easy to see all these tools and expect that miracles can happen.
It's not just the development, though, it's the design work. Creating designs that don't impose constraints on other elements of the work (meaning designs and previous code have to be completely uprooted). Designing lobby and general mechanics to work well in the early stages of a mod's life, when there isn't a critical mass of players available. Performing analysis to find play elements that turn out non-sane before the devs commit time to earlier, faulty specifications. Organizing a large development team. Maintaining high-standards of development and effective technical documentation.
It's a really big thing.
