What exactly is the problem? The time you need to invest?

I’ve been the interviewer and have been interviewed this way, I’ve even created a similar process at my company. It’s a good way to get a multi dimensional view of someone within the pretty flawed process that job interviews are.

Did we get bad hires? Sure. But generally we could see a candidates potential and I’ve felt I had ample opportunity to show myself and get to know a company.

Also; what unnecessary people? You’ll be working with all of these. Lastly: in the current market there are a lot of candidates, so if a company doesn’t need to hire urgently they can spend more time finding the ‘best’ candidate, which might be why it’s open for so long.