Especially at senior levels, there's ramp up time AND a larger scope of impact if the person makes bad technical decisions or negatively impacts the team. A bad hire is super expensive on both those dimensions. It's even worse when the candidate is hired into a leadership role and will be responsible for further hiring.

It's easy to see if someone is incompetent during their first 90 days: that's a basic recruiting failure. It's the other type who are sort of good, but ultimately disastrous, that cause real problems. And often, these mis-hires take ~12 months to identify and correct. The result is that caution dominates when hiring.