Procurement is a nightmare. The first operational jet fighter, the P-80 Shooting Star was delivered by Lockheed in 143 days.
Well, that was back in the 1950s right? Systems today are a lot more complicated. But I still think we should be able to field new jets in 5-8 years, not decades. The acquisition process we have is very cautious and, as a result, stretches out the process to an almost absurd time frame.