Unlike conventional businesses where a good or "binary" service (it works or not) is sold, advertising is a much more nebulous good whose efficiency can't be accurately measured. This means there are tons of inefficiencies where middlemen can skim something off the top:
* a product manager decides to include ads in some digital product. Their analytics show plenty of "engagement". The engagement is actually people accidentally clicking on the ad while hunting for the tiny "close" button, but even if the PM suspects it, they have no reason to volunteer that information. They keep getting their salary paid and even earn a promotion based on the engagement numbers.
* the developers are tasked with implementing the advertising infrastructure - they get paid while padding their resume about how they're building "scalable" systems.
* the "scalable" system runs on a cloud provider and earns them a ton of money. Cloud provider is happy.
* some marketing agency is given a budget to go and spend on ads. The person there maybe even knows that advertising in the aforementioned product is a bad idea because most of their clicks are fake... but if their client is tasking them with burning money, why would they refuse?
* a marketing person at a big company that doesn't actually need any more advertising to succeed is given a budget and spreads it across a few marketing agencies including the aforementioned one. They get paid, why should they refuse?
At every layer (and I haven't even listed them all), people get paid by skimming something off the top. It doesn't matter whether the advertising works, because nobody in the chain has any incentive to admit it while the status quo is so lucrative, so the rational thing to do for everyone is to not rock the boat.