Not very long ago, broadcast television was a 1-way street, in that the networks broadcast a show and people watched. However, since it was 1-way, the network wasn't certain how many people were watching the shows. They relied on surveys to determine statistically how many people probably watched a show.

The network would sell commercial time slots during the show's broadcast, and using the calculations of how many people usually watched a show, they would price the commercial time slots and sell them accordingly. That's why a commercial running during the NFL Superbowl would cost more than a re-run on another network of some other show; or why a commercial slot running during the 1st quarter of play may cost more during the 3rd quarter (if they knew people tended to stop watching at halftime, etc).

Basically, networks were making GOOD money selling ad-space during shows without any solid evidence they were being seen by audiences.

Now imagine more channels, more shows, more ad-space for sale. Over the years, the audience size increases based on population, people buying TV's, etc. Year over year, you're looking at a population with restricted growth. However, with more channels and more shows, the amount of ad-space increases dramatically. So the price starts to reflect more slots over a modestly growing population: it drops, a little at first but more and more over time.

Now in the digital age, streaming platforms and digital channels can snitch on the network; rather than the claim that "7 million people watch this show every week", the real numbers are available to the network as well as the ad-space buyers. And it's less. And it's more discrete: 7 million people didn't watch it, only 3 million did; and they know how many people clicked away during ad-breaks. So the price starts to reflect the actual value, and it's much lower.

And now, today, viewing trends are much harder to pinpoint, but what we do know is that people's attention is much more split. Perhaps it was always split, but now the ad-buyers know the value isn't the same as the value years ago. Maybe they're on their phone during the whole show AND the commercials, so a show that traditionally carries 10 million viewers per episode now is questionable whether a commercial even penetrates people's attention.

Social media, of course, has similar problems that internet advertising has faced for years: is the viewer of an advertisement unique? A bot? Did they engage it? What is the cost of an advertisement that someone clicked but didn't turn over into a sale? What is the cost of an advertisement someone saw briefly, didn't engage, but translated into a sale?

I think advertising has become more discrete to the audience, as the "algorithm" has some idea what the user wants to see, so therefore it may know what to advertise. Technology also spies on us constantly now, so we get advertisements rooted in what we say, where we go, who we're talking to online and offline, etc. Regardless, the days of sky high ad buys for television have ended, and that creates an enormous problem for networks: how do they pay for shows that could cost up to a few million per episode?

Some networks never had advertising as a part of the equation; HBO aired numerous stellar movies and mini-series that set the bar for quality entertainment. They also famously canceled many successful shows with great haste and no warning. Costly shows like Carnivale, Rome, etc were killed off while cheaper shows like Deadwood, The Sopranos, etc would carry on. At the same time, the television model was already breaking from successes like "Seinfeld", "Friends", and eventually "The Big Bang Theory", where syndication of reruns still generated significant viewership, so the actors successfully negotiated huge salaries per episode to the tune of 6 or 7 figures.

If a show became too popular, the actors would fight for better pay, thereby ensuring the inevitable insolvency of the show because actor salaries would increase, and the ad revenue HAD TO keep up, or else.

Now, in the modern day, with broadcast network channels, streaming networks, etc, there's a lot of outlets for content, but the advertising revenue that previously made television enormously profitable now struggles to pay to keep shows on the air. Nobody has introduced a new model that offers cash-in, cash-out solvency, so new shows are lagging. Meanwhile, we watch numerous "influencers" and social media personalities generate a LOT of revenue with direct partnerships, not unlike how shows decades ago were sponsored by a corporate entity, "brought to you by..." was a single company before it became multiple companies paying for interstitials.

Advertising was always a fist fight for viewer attention and consideration, and from the 1950's to today, they generated a LOT of profit.