The medium to broadcast ads has simply shifted over time. It used to be expensive tv ads, billboards, and print. All very expensive and you could not target population segments precisely, it was limited to what time or during what tv show the ads were shown, or in print political leanings. Retargeting was almost impossible and expensive.

Then people started watching less tv and reading less newspapers(or did so online). Then JS got mature enough to track users and then the marketing departments started instructing the devs for maximum efficiency. At the cost of user ui/ux. Almost every web page loads ads via ajax , consents popups, email list ctas with a lot of bloat. Nobody knows how to fix it, or the financial incentives are just not there.