I believe the issue is that this can no longer be a horizontal play. MTurk was mostly for unskilled tasks...the kind AI can do well enough that it isn't worth the cost differential to verify it or keep farmed to humans. The "trust but verify" AI output is now the kind that requires domain expertise. This is what most full stack AI companies are bringing to industries.
Curious if this kind of work will come around again one day or was just a moment in time. If it does I'm sure it will be specifically about generating training data.