As much as I love a good conspiracy theory, and yours is pretty damn good because this industry has colluded to keep wages down, I'm hard pressed to believe that this is the case. Interviewing is hard. Startups are led to believe they need to have a higher rate of false negatives over false positives to ensure they don't get a "bad hire". Most people in this industry just follow the trend they see. It's all they know. They have never seen another successful hiring strategy. So it persists.