I don't know if it's a majority, it may be, but it's at least a large minority.

Airlines (both passenger and freight) don't want to pay to train their own pilots, so their staffing pipelines rely heavily on ex-military pilots.

Many military and passenger aircraft share a basic airframe, for example the KC-135 is a 707 configured as a tanker for aerial refueling. Even the instances that are not - it would be relatively straightforward to transition from, say, a C-17 to a 767 or something like that.

So pilots tend to retire from the USAF and fly the friendly skies with civilian airliners.