Yet no one claims using the MTURK API helps others "develop employable job skills"... website testing, cheap content writing and data entry aren't exactly the types of skills employers are unable to find when hiring.
As mentioned in other comments, most trainees are working on basic technology skills rather than specialized job training. This is a place where even simple work like data entry, web research, and software testing can provide a useful opportunity to practice skills.
This means you end up running it repeatedly to get the right results, engineering safeguards against malicious or underpaid workers, and more.
Because MobileWorks is managed and not a marketplace, you pay a fixed price with no engineering cost.