Because said site got popular, and diluted, would be my cynical guess. Your observation is spot on, though.
By your own admission, advertising has a poor signal-to-noise ratio. The mental cost of advertising (not to mention the economic cost) is very high[1]. Why should we stick to a system with such a poor efficiency?
> I want to buy
There are other ways to do that. For example, the internet should make it very cheap to make information available to anybody that wants to buy something. Services like google shopping are very primitive attempts.
> making people aware
It's the "making" part that many of us find extremely unethical. Like everything else, consent is opt-in.
[1] If you don't believe there is a high cost, isolate yourself from ads for a month or two. Humans are very good at adapting, so it's easy become blind to how much energy and mental capacity are necessary to filter ads.
Yes, I thought it was ok, but she had to hire a babysitter and in the end, if I'd been asked prior, I would've said no, or asked for a better movie (like The Martian).
I have the feeling ads almost always sell you something you'd have been better off not buying anyway.