Personally, I dont think it's insurance. Many legends have their roots in some kind "insurance requirement" but it's largely not true. What about those that go on long term sickness, and come back to fly? From experience, I have been at two fairly busy and large charter outfits that hired people who had not flown for in some cases 10 years and others who had not flown the type in 10 years. Supply and demand is usually the primary dictator.