This model exists, although not yet in mobile - Solve Media does it as a way of monetizing already-existing captcha traffic. Instead of filling out a captcha at the end of a signup, you watch an ad and type out a phrase within the ad to move on.
I can't imagine this would perform well in places where the captcha wasn't already required - there'd be too much dropoff. So while it might be interesting to advertisers, no publishers would run it, unless you charged so much that it made up for that conversion dropoff, which would then cause it to no longer be interesting to advertisers.
Making an ad product that keeps both parties happy can be tricky.
I can't imagine this would perform well in places where the captcha wasn't already required - there'd be too much dropoff. So while it might be interesting to advertisers, no publishers would run it, unless you charged so much that it made up for that conversion dropoff, which would then cause it to no longer be interesting to advertisers.
Making an ad product that keeps both parties happy can be tricky.