1 To increase revenue
Many websites depend on ads to generate revenues. The admins of those websites might increase the number of ads in order to increase the sites' revenues.
2 To increase clicks
Most ads that websites display are click-based. For a site to generate good revenue, the rate of ad clicks must be high. Webmasters might put many ads to increase the possibility that someone would click. (See Why do websites disable right click?)
3 The site doesn't sell anything
Sites that don't sell anything are more likely to contain ads since the webmasters will find no way to make money but to put ads.
4 Large sites don't get a penalty
A large and well-known website can hardly get a penalty for the ads because of the site's overall quality. Because those large websites can get away with it, they feel safe putting more ads.
5 Ads are the simplest way to monetize
Ads are the simplest way to monetize. A webmaster who doesn't have an idea how his site will generate money can just put some ads until they decide how they are going to generate revenues. (See Methods for increasing client conversion online)
6 Click rates are going down
Click rates have been going down since the smartphone revolution started. To solve that problem webmasters have to put more ads.
7 Banner blindness
People get used to banner ads and stop clicking on them as a result, which is called banner blindness. To solve this problem webmasters keep adding new and different kinds of ads to their websites.
8 Ad providers create new units
Ad providers such as Google AdSense keep creating new ad units that have different forms and shapes. The ad providers encourage webmasters to put more ads on their sites.