Let me take you further in this direction of thought- many offers aren't accepted because the candidate, after having been through a dysfunctional interview gauntlet, realizes a different opportunity is better (sometimes after waiting weeks to get the start date).
If interviewing has been skillfully conducted, and the candidate gets an offer in 24 hours, there's substantially less likelihood they'll turn it down, because they won't have had as much time to interview elsewhere, or wonder what organizational deficit is causing the delay.
That is certainly true. Since you had already jumped to implicate the interview process as the reason it took so many candidates to make a hire, I was simply offering an alternative. In fact, it could be that both the interview process and the offers are flawed; there can be any number of reasons why a candidate doesn’t convert to an employee, and you’re solely focused on one.
If interviewing has been skillfully conducted, and the candidate gets an offer in 24 hours, there's substantially less likelihood they'll turn it down, because they won't have had as much time to interview elsewhere, or wonder what organizational deficit is causing the delay.