Not only does this disincentivize HR from running fake vacancies or stringing multiple candidates on just to keep their options open, but it also solves the problem of unemployed people job-searching effectively working full-time for free. The fact that companies would have to pay to hire workers would mean they try to make the selection as short and effective as possible.
That’s already a flaw of the current system, so no change means no new downside. People receiving unemployment usually have to prove they’re looking for work, but there’s not usually a requirement that you’re applying to things you’re likely to get.