The real reasons
This mess might not exist were it not such a royal pain to employ people directly. It's like fly-tipping: if you could get easy pickup of your old stuff and rubbish, why would you resort to using cowboys, or to driving yourself out into the sticks to tip it into a ditch?
One thing is certain: once IR35 takes over, there will be some new shenanigans to reduce the cost and bother of hiring people. And the Revenoo will plot new depredations.