Why Equities: Bonds ratio is 60:40? But why?

I would guess that Equities swing up and down. So fixed income swings other way around and 60:40 sort of event it out. But this is a very simplified approach. I sort of expect that if equity (broad market index) cash, they will eventually come back up. So during the time between equity swing down and come back up, I need a way to withdraw funds without selling equities. It is also expected that if equities crash, feds eventually (1 year from the crash) will come to the rescue and try to lower in