Google is a hundred-billion dollar company precisely because it allows advertisers to see how effective their cost per sale (or action) is, compared to TV or newspapers, where advertisers are flying blind.
https://www.quora.com/How-can-I-reconcile-Facebook-advertisi...
Likewise, many advertisers aren't doing brand campaigns, they're doing direct response, that is, trying to get people to directly click to their site and convert.
Finally, advertisers on video sites tend to pay per "completed view" for some definition of completed view, usually at least a significant chunk of the video and not just the first few seconds before the skip button appears.