Why is this? Is it because it is easier to categorize users as opposed to content, or do ads perform better when targeted to specific audiences? If I had to gue

Why is this? Is it because it is easier to categorize users as opposed to content, or do ads perform better when targeted to specific audiences? If I had to guess I'd think that audience targeted adverts would perform better, but here we have a large amount of companies upset that their adverts are appearing next to specific content.