I don't think you can sensibly say that most interviews are making people "work" for free- you're not going to get any productive work product out of an interview unless you're literally making someone do a trial shift in a coffee shop, which I think people would agree is scummy. That doesn't really happen in Tech. I can pay you to do our coding test, but I don't think anyone can reasonably argue that 350 different solutions to the exact same toy programming problem has any value to my company. Also, what am I going to pay you? Minimum wage? £20 or so? We could do that, but I'm not sure it would make any difference to anyone but the most marginal candidate.