Is a catastrophic event likely to happen? No. But if it does and you "self insure" you can pretty much guarantee to wipe out the vast majority of the average American's net worth. I couldn't imagine making mortgage payments for 30 years then having to carry enough investments to self insure and not be properly fucked if something did happen. Very very few people have the assets and can carry that risk.
A bunch of people gave me the business about health insurance in another thread, but I've seen the benefit, multiple times mind you, of having decent insurance. I happily write that check.