Most people who read the original article will never see the retraction. So they get all the page views and ad impressions from running a sensational story that

Most people who read the original article will never see the retraction. So they get all the page views and ad impressions from running a sensational story that goes viral, most of those people will continue to believe it, and even if some do find out it's false there's this idea that they shouldn't even look down on WaPo for this because they eventually retracted it. It's win-win-win for them!