Search engines.
The objective is to get certain things to show up in Google/Bing/other results so people click on them there.
Shoving them onto high traffic sites with decent credibility is a quick way to achieve that.
As for adverts that are unwanted - clickety click on those a few dozen times to increase costs for the advertiser.
Advertisement sellers (guess who is the biggest of them all) are interested in automatic clicks from bots. More clicks, more commission. It may not be the spammers who are the problem, even though you blame them. Same like you would blame short sellers in the stock market for a collapsing investment, while the people taking your money are someone else.