As opposed to folks like Eisenhower or Reagan (Army) or Carter or John F Kennedy (Navy?). And for the record, George HW Bush and McCain were Navy as well.

Bottom line is that up until George HW Bush, we're talking about folks who would have been around in WW2. The US had conscription, so as a general rule, you could expect that unless there was a particular skill, that person would be in the military. Granted, it wasn't all conscription: Ike and Reagan, were in the military before the War, and Carter volunteered for Annapolis. And of course, McCain and GWB were both Vietnam era, so volunteered, but there is also the matter that in the US, military service is something of a family tradition. (Which itself has led to interesting demographics over who joins these days and where they are from)

Until recently, some form of military service was also seen as quite a positive attribute in politics, so many successful politicians would have run with their military background as an asset.I think you might want to re-verify the assumption on your question first, though.