What happened was that pageviews gained monetary value, by way of advertising. Each and every one of those low quality pages you refer to is plastered with ads

What happened was that pageviews gained monetary value, by way of advertising. Each and every one of those low quality pages you refer to is plastered with ads. That is the very reason for their existence. Of course, the old web wasn't perfect, but it was very different. What's surprising is how few the holdouts are. Wikipedia is one of those few.