The short answer is expense; The US has two such programs in development (both called NGAD) and they're likely to be the most expensive aircraft ever developed

The short answer is expense; The US has two such programs in development (both called NGAD) and they're likely to be the most expensive aircraft ever developed by the Air Force and Navy. This 70 minute talk covers all of the current 6th generation fighter programs in development and a major running theme is extreme cost. https://youtu.be/RPrWm6fWuaM https://youtu.be/RPrWm6fWuaM