Because you're forgetting the other side of the equation, what sort of people are doing mechanical turk?

Are they 1:1 your peers, colleagues?

Or are they underpaid, most likely emerging economy countries, and very overworked labor that you're paying for?

You can browse to the mechanical turk subreddits and you'll hear the other side of the table that they spend 15-20-hours on a task to get what, a few dollars?

Pay more, get better results. The system itself is designed to exploit.