You're not considering the situation where the candidate has a job already but is looking for a new one. Quitting your current job is a large commitment, and doing so for only a week trial period (where the odds are higher you'll be unemployed afterwords) is a pretty large risk.
That being said, for someone who is currently unemployed, putting job interviews on hold for a week is less of a risk
Ah, indeed, I did not think of that. There'd be vacation days to avoid having to quit your current job for that week. Still, there's other problems, like maybe the contract forbidding you to have other jobs unless cleared by your employer, and taxes because you suddenly have two jobs – not worth the hassle for a single week, I guess.
If I'm looking for a job so desperately that I'd be financially devastated by a lost week, then I'd surely jump at a paid week of work, no?