Right, but these companies have lots of customers. They just get them from one place.

It's like renting a particular location: it's technically true that you could lose your lease, but that's only a risk to your business's long-term viability if your lease is artificially cheap (otherwise, you'll just move to another place on similar terms). And if your location is a particularly good deal, you should take it, even though losing it would suck.

Online, the same dynamic applies, but with lower transaction costs.