Because fighter type aircraft who are expected to make high-g maneuvers reach high angles of attack when doing so. A dorsal mounted inlet would be starved of air and could cause a compressor stall.
Stealthy aircraft that aren't expected to make high-g maneuvers do use dorsal air inlets, such as the original stealth demonstrator Have Blue, and the B2 and B21 bombers.
The North American F-107 had a dorsal intake, and the pilots feared (needlessly) that if they ejected they would be sucked into the engine.
