People are lazy and will pay money so they don't have to think about licensing. If you are a graphic designer working in an ad agency and you have a choice between "go online and download a PD work and not be sure" and "pay Getty $50 and if there's a copyright issue, you can tell your boss that you paid Getty", which do you do? Pop it on the corporate credit card is the easier option.
It's the same reason big companies were suspicious of open source for a long time. And some still are.