Part of what makes federal agencies effective is their relative isolation from the public and their quasi-meritocracy structure. While members of Congress are certainly intelligent and capable people, I don't believe they have the qualifications to determine what is and isn't necessary for a federal agency that operates outside the purview of their knowledge. If NASA says they need 2.3 billion dollars for ramjet research, how many members of Congress will even know, in detail, what a ramjet is, let alone why it's important?