There's a cost (in terms of time & money) to onboarding someone, integrating them into the company and team, and getting them to be productive. That's a reasonable investment for a long term gain, but for a short probation, it rarely pays off for the organization. The churn itself just isn't worth it.

Moreover, it means the candidate is in an uncertain state. Should they keep interviewing if it doesn't work? Should they change their lives and relocate? Etc.