You are right that military experience is not per se required. But all of those people you mentioned had deep experience within the defense department and/or intelligence agencies. Panetta was CIA director, had been in the House, WH chief of staff, OMB -- Panetta knew his shit. This assclown is no more qualified for the role than me. Actually, probably less qualified, since I can get a security clearance and he cannot.
There's no "relative lack." It is a complete and utter lack of any relevant experience whatsoever. He's never run anything. He has no demonstrated skills in the work that a Sec Def has to do. Imagine this assclown trying to manage an $800+ budget. Oh my God.