Howzabout just dusting off the YF-23?
Making a guess, as a non-professional....
Because it was only ever a prototype and never went through EMD and was built in 1990 so the team which designed it would now be in their 60-70s. The engine it was built around no longer exists, nor the electronics or software which wasn’t open architecture.
The stealth technology was the same costly first generation type as used in the F-22 which would need to be replaced, which would lead to a complete airframe and shut line redesign. (e.g. 3D laser sintering of metals such as titanium will allow major weight saving and redesign of structural,elements - which change stress loads leading to further changes etc etc)
All the above, plus incorporating modern composite materials would almost undoubtedly make it more expensive and lengthy than starting with a clean sheet design, because you’d have to work out what you had before working out it could be modified into what you want rather than just starting anew.