The ads that follow you are called retargeting and the reason why they are used is... because they work! They get dramatically more clicks and conversions than other ads. There are also less obvious retargeting ads where websites trade their audiences with advertisers.

But yes, the ad system is completely broken. It's a complete security nightmare - they track you, in plain text, without telling you, they sell data about you without your consent, they are a great way to deliver exploits to millions of people (and their security policies are a complete joke). There are click-farms everywhere, pricing is not transparent, there are less-than-honest practices against advertisers all over the place, inventory gets sent to networks where they don't belong, then there are the intrusive ads, the dark pattern ads...

It would be nice to have an online ads industry that is safe and secure and honest to fund Internet content, but so much about (mass) advertising is so inherently anti-customer that I don't think that will ever happen. I hope we'll figure out ways to directly pay for content instead.