I don't know about others, but with Amazon specifically, it's always been very clear that their "losing money" in aggregate was purely on paper, for tax purposes: their ability to undercut everyone else was initially based on being online without the brick-and-mortar costs that other stores did, then on economies of scale, and now on being the 900kg juggernaut that just has more money than God and can blow it on running you out of business if they feel like it.