The reason TV adverting was expensive was that slots were rare and there was far more demand for the good slots than there were slots. The ad guys did a great job showing the world they were good honest people who were helping their clients sell stuff. This is why the honest guy who would never do any wrong in Bewitched in the 60s was an ad man. The show was there to sell more advertising. The clean ad man image has changed in recent times to a image of a person who can manipulate anyone into buying a worthless product but that too helps sell more TV advertising.
The rarity of slots did matter in the early days of TV and as a result, several industries were changed. In the early days of live broadcast, only car dealers near the TV stations could go live but people were so impressed in the first months of those ads that they did work and and other dealers would buy larger lots near the stations and created a new type of car dealership. Those dealers grew rapidly, not so much because of the ads but because of the influx of new buyers as the two car family became the norm. The TV ads didn't tip the buyers, it tipped the dealers into buying more ads. Even today car advertising is a significant part of tv station income.
Remember that the customer of the advertising is not the final consumer who buys the product, it is the marketing department of a large corporation who pays for commercial tv and the advertising business has many ways to prove their ads work even when the sales figures show they might be running off customers.
Now the internet ads are over saturated, they aren't worth anything yet idiots keep paying lots of money for useless ads because they think they work. Even google is playing the old tv ad game with their analytics package which helps show businesses how well the ads are working. Too bad that details slip through like you pay for a specific key word and you find out that most people are trying to avoid results with that word but using google improperly so you pay $5 per click for people who will never buy your product.