I think the fraud is asymmetric. It's relatively easy for me to hire someone to perform the work the employers wants for X hours one time (i.e. interview), but relatively difficult for me to subcontract it reliably for 40 hours every week.
This asymmetry exists regardless of how representative of real work the interview process is, short of a full-time engagement.
If your interview isn't screening for the work you want somebody to do then it's not a good metric for finding successful employees.