You can have 20 years of experience or 20 one year experiences.

You can also be straight up lying about having 20 years (or any) experience. Most people won't be doing this, but it seems like a basic feature of a hiring process to have some kind of check against this.

Also common is someone with 20 years at the same company that's has been coasting. Maybe they still work on a decades old enterprise app and have no relevant recent experience.

Lots of jobs aren't all that demanding.

The check is what the candidate says, isn't it? If the candidate lies, you will know the day you hire them

Isn't the idea that you know before you hire them so that you hire the right person?

Yeah, but you know when you obtain the information - which would be reading it on their resume or talking through it with the candidate. Asking them to confirm what you've been told is my issue with this whole thing

When asking someone with 20 years experience to do junior level problems, you're likely measuring something that devalues the experience tremendously