I am a hiring manager. I've also put significant thought into how we can change the hiring process, if only to get better candidates. However, it comes down to this: I only get 2-3 hires a quarter. Trying to get a meaningful result on changes I make is difficult. If one doesn't work out, did it not work out in a way that my original process would fix?
And so, we get conservative because even though it rejects a lot of qualified candidates, it also gets good candidates.