I think part of the point may be to select for people who don't consider "being interrogated" to be a contest. There are plenty of folks who are just like "Here's what I know, here's what I don't know, you decide whether you would want me on your team. If you don't, that's fine, I'll find another team to join." Then there are other folks who, when they're asked a question and don't know the answer, feel like their self-worth is under attack. The latter group can be really hard to work with, and can do a lot of damage to a team.

I had a PM friend who would try to push the bounds of a candidate's knowledge until they gave up and said "I don't know". The actual knowledge wasn't the point of the question: it was that they could say "I don't know", because PMs who can't tend to make life miserable for the engineers & other teammates who work with them.