Why are interviews harder than the job?

I know it is hearsay, but it seems many times interviews try to get free work from candidates. Here, have a look at this real problem, see if you can solve it in whatever the interview time frame is.

When there are too many candidates and not so many jobs, and absolutely no regulation, employers are free to exploit applicants without consequences. Employers will do this even when there is no actual job being offered, so they can use the availability of applicants to apply downward pressure on the salaries of the workers they already have.