Because both its budget and operations are secret.
Actually, the CIA and the FBI are coops. The CIA is able to work internationally (gaining "intelligence" which is another word for "information" - so they are the spies) and the FBI is restricted to US soil (which currently also includes Countries the USA "invaded" to engage in "war-like operations").
The CIA has a history of being almost stronger than the US president due to the powers it gains from it's intelligence work.
To keep it short: someone was sleeping years ago and now, they are to smart to "keep them checked".