It's not about "Can the candidate solve this mission critical problem we'll spend a week on, in an hour?". It's more like "Can the candidate recognize a graph problem?". If they can't even tell that this is a graph problem, how are they going to know that today is the day we need to ask someone for help, and think about this very hard?
An example: Many times I've asked something like Word Ladder[0] to a candidate, and they can't tell that this is a shortest path problem, nor use the common tools to solve such things. Even with hinting and helping as much as I can, sometimes they just don't have those tools in their toolbox. And my job is to find out whether they do or not.
[0]https://leetcode.com/problems/word-ladder/ and no, I don't use this exact question.