MIL-SPEC is a very broad category. It just means it conforms to a military specification, which can be very simple and easy to meet in some cases. But when the MIL-SEC refers to safety-critical and flight-critical specifications it's a whole other story, and does become an extremely costly and difficult to meet set of standards for good reason.
Compare the cost of a MIL-SPEC ThinkPad with that of an actual flight control computer designed to be an integral part of an aircraft's avionics.
Compare the cost of a MIL-SPEC ThinkPad with that of an actual flight control computer designed to be an integral part of an aircraft's avionics.