> Can't they just let people go who interview very well but are poor performers on the job?

Eventually, yes. But it takes time for them to start, time and money to onboard them, evaluate how they’re ramping up, then if not acceptable, to follow whatever performance management process is indicated by the company and local law, then transition whatever work they were doing. This could be several months and tens of thousands of dollars just to get back to a worse state than when you walked into interview that candidate.

Interviewing even slightly better than last year can pay large dividends.