Publish or perish.

Academic publishing is in a very weird place and is very, very political. Its true that authors have to pay to have their papers published in most journals or conferences after they've been accepted, but like all things academic, this is highly dependent on the field. Some universities will reimburse professors publishing costs, others need to pay out of pocket or with grant/public funding.

While its true that there are open-access journals and conferences without such costs, I would wager that most well known researchers would avoid such avenues of publication due to prestige. The larger journals and conferences have review boards where the top scientists in the world sit on them. As a potential published author with such an outlet, its a great honor to even be considered. Most researchers don't want to take the risk of going with a less prestigious outlet if it will run the risk of smearing their image or damaging their ability to publish in better outlets in the future.

Source: Was a Doctoral candidate that ran the whole ringer besides the dissertation.