Depends how you count and where you are. It’s true that most of the time you’re mostly going to see CPM ads, but CPC and CPA ads are quite popular on some networks (I’m pretty sure, for example, that most ads on Facebook are CPC or CPA, not CPM).
However we should be clear on why CPM is popular: it’s predictable. If you’re trying to spend $10k on ads, buying CPM ads is simpler because it works exactly how you think. Yes, you might serve ads to bad inventory, but at least you served the ads and the money was spent (why might the money being spent matter? if you’re an agency spending on behalf of a client and need to hit a spend minimum; principal-agent problems abound here). CPC and CPA make it much harder to ensure all the money gets spent: maybe your ad sucks or there’s not enough inventory of people who would click your ads to meet your desired spend. So CPM still fills the gap.
As I hope this makes clear, lots of ad spending is wasted. In many cases you can’t really get more results from spending more than, say, $10k/month unless you’re in a zero-sum battle for brand awareness: at that point you’ll have already saturated all the inventory that will generate results for you, and additional spending has close to 0 marginal return. Of course, marketers want to spend money they can plausibly claim produced results, so more money gets spent, often in ways that are decreasingly easy to trace effectiveness of. But now we’re getting way off topic.
Depends how you count and where you are. It’s true that most of the time you’re mostly going to see CPM ads, but CPC and CPA ads are quite popular on some networks (I’m pretty sure, for example, that most ads on Facebook are CPC or CPA, not CPM).
However we should be clear on why CPM is popular: it’s predictable. If you’re trying to spend $10k on ads, buying CPM ads is simpler because it works exactly how you think. Yes, you might serve ads to bad inventory, but at least you served the ads and the money was spent (why might the money being spent matter? if you’re an agency spending on behalf of a client and need to hit a spend minimum; principal-agent problems abound here). CPC and CPA make it much harder to ensure all the money gets spent: maybe your ad sucks or there’s not enough inventory of people who would click your ads to meet your desired spend. So CPM still fills the gap.
As I hope this makes clear, lots of ad spending is wasted. In many cases you can’t really get more results from spending more than, say, $10k/month unless you’re in a zero-sum battle for brand awareness: at that point you’ll have already saturated all the inventory that will generate results for you, and additional spending has close to 0 marginal return. Of course, marketers want to spend money they can plausibly claim produced results, so more money gets spent, often in ways that are decreasingly easy to trace effectiveness of. But now we’re getting way off topic.