People's experience can never be boiled down to a number.

Like it or not for hiring purposes it does.

Well, it is not useful, then.

It does not work very well for new grads. In grads case I question the purpose of interviewing at large. They studied, completed one goal, all we have to do is give them the tools to understand their next steps. With grads I focus more on their interests, what they are reading on the subject matter domain (data in my case), etc. There’s no really silver bullet and the value of this question even with experienced hires is the follow up where you drill down on technical and behavioural elements of what made this project/product the one they chose to discuss.

No numerical scale, you either want someone in your team, or not.