Because jack of all trades never works... Except for the P-51, Spitfire, F-4, F-16... And heck, lots of other fighter/attack planes.
None of those were designed as jack-of-alltrades. The Lee Begin (of Northrop) philosophy was: design uncompromisingly for air superiority, and you will be surprised what else the lean mean machine can do. Add the bits beforehand and the result is master of none.