Probably comes from the vague mission, unlimited resources (yes they are a curse sometimes), single super power winner mentality, lack of imminent treats for the us mainland, and lack of political will to do anything.
Unfortunately, the history of trying to make merged designs isn't great. Differences in missions (and, frankly, the fact that those missions have been changing in many cases) seem to lead to designs that cost too much, don't do most tasks especially well, and take too long to design and build.
While it developed some useful things, and pioneered variable geometry (swing) wings, it was almost entirely a debacle. Forced the Air Force to use designed for the Navy F-4s for a long time, which I'm sure they hated (and of course it was obsolescent, required two crewmen, the engines smoked (makes it much easier to spot in ACM), etc. etc.).
I wouldn't be surprised if the Air Force will be forced to use F-18 Super Hornets to replace falling apart F-15s. A nasty failure in the process of going from the design drawing to what was built in the factory resulted in all of our air superiority variants having a nasty weakness in the attachment of the cockpit to the air frame: https://en.wikipedia.org/wiki/McDonnell_Douglas_F-15_Eagle#S... (living back in Missouri at that time it was pretty big news that one simply fell apart around its pilot while doing pretty much normal stuff).
JSF -- a mess
JTRS -- (Joint Tactical Radio_Systems)
etc.
Given that the original concept was strictly a daytime fighter I'm not at all sure the decision was incorrect.
http://en.wikipedia.org/wiki/General_Dynamics_F-16_Fighting_...