Most of these sites will have very detailed monitoring of performance/revenue generated by their ads. In all likelihood they've A/B tested ad impressions, click

Most of these sites will have very detailed monitoring of performance/revenue generated by their ads. In all likelihood they've A/B tested ad impressions, clicks, conversions, and revenue generated by a single-page article versus the same on a paginated article, and discovered that (intuitively) spreading the content out over multiple pages with more opportunities to display ads gives better results. Maybe it's only marginally better, but in the world of online advertising even the tiniest improvement in conversions can mean huge changes in revenue when you throw enough users at it. The fact that some users may be annoyed by or made "immune" to ads is moot. They'll go where the money is.