Depends on what type of ads you are referencing. But I'll assume you are talking about reputable ad services.
Now I'll preface this a bit, while I work in the ad world, I don't work in buying or placement. In fact, I've been the devil before that makes those god awful ads.
For starters, why would I make an ad even though I hate them??Money. I can work my ass off for two months building a website, or kick out an ad in 6 hours and make the same money. Freelance of course. On salary I'd shoot my brains out in a few weeks.
Now, why are ads still a viable medium on the internet? With the premise that "no one looks at them?"
Keep in mind, ads are sold by ad agencies. Ad agencies are only as good as their talk. So before we look at the true convergence factor, we have to look at impressions. "We served up 10,000 impressions. Even though we only got a click through rate of .097%, we made an impact on 10,000 site loyalists...blah blah blah." In the ad world, impressions matter. When you go to a concert and [enter big brand here] sponsors the concert they aren't doing it out of the goodness of their heart. The ad agency behind that idea is putting metrics against the impressions, real or assumed.
Now onto the real(ish) facts. Say a company wants to convert IT professionals to use their new cloud services. They will tell their media planning agency (also generally called an AD Agency) to target the IT Demographic that has a certain skill level, business need, and educational interest. The planning agency then RFP's (request for proposal) a bunch of websites to see who can deliver. They then pay $10k - $100k (or more) for a set number of ad placements. Once all the ads run (and a whole bunch more BS fun they have to deal with), they look at all the downstream metrics. It's not really about clicks anymore. How many people signed up for more information on the landing site? From that, how many converted into new customers?
Simple math: (faux numbers)
100 ads served. $10 per ad.
20 clicks.
10 email addresses submitted.
1 new client.
So you paid $1,000 to gain a new client. Now if we are talking about IT services, that new client could be worth say $50k a year.
Worth it.
But what about you? You get "road blocked" a few times a day. That's when an ad blocks the website content until you hit close. I've seen click rates as high as 20% on major sites, with a signup rate near 40%. You might hate ads, but when [coffee shop] has a free drink coupon, you're f'n signing up.
Besides, what do you think pays for the web? Hate to burst your bubble, the site you are reading this on is owned by one of the largest publishing companies in the world. Those ads you click, pay for this.