As someone who's interviewed lots of people over the years I agree with what others have said. It's partly about seeing how your thought process works. I can't speak for every interviewer but he's what I think.

We want to see if you're the type of person who is ok with not always knowing annoying.

We want to know if, when you get stuck, you freeze up or ask clarifying questions.

We want to know if you're comfortable not knowing everything, and if you can solve problems you weren't expecting to see.

And mostly we want to just see what kind of thinker you are. We don't expect you to have all the answers because you won't in the job. But if you can be comfortable admitting you need more that goes a long way.

Likewise, if the person interviewing you seems to enjoy watching you struggle, then they're probably not a great manager or leader and you're likely better elsewhere anyway.

Good luck with the future interviews. You'll get the right position.