The web thrived for a decent number of years with ads before it began to really spiral down.

For me the biggest cause of that spiral was the shift from most users using desktops to most users using mobile to interact with google.

In the desktop search world, google used to respond with large comprehensive articles on subjects that not just answered peoples questions but also educated them on the subject around their question. This was the golden age so to speak, it answered your question along with 10 others on a subject but also helped you to understand if your question was even the right one in the first place.

With mobile, google shifted to trying to directly answer the question you asked and nothing else, presumably because people were far less willing to read large articles on mobile and just wanted fast answers.

This latter approach is much easier to SEO garbage than the original that took real knowledge and effort to write.