Why are the intakes of some fighters not attached to the fuselage?
That's called a 'fuselage boundary layer diverter channel', or splitter plate.

The air near the fuselage or wings is slowed down (thereby losing energy). Those plates ensure the jet engines get free stream air, which improves the engine performance. This free stream air will be undisturbed, allowing a more uniform intake of air.

You can also find it on the Concorde.



Related: What is the function of the holes in splitter plates?