They don't know what the interviewer wants to hear. There are places where every admission of not knowing something is held against you.

If the employer would hold that against you, it's not a place I'd want to work. Not sure about you.

FAANGs are notoriously famous for having one bad feedback rule. That is, if even one interviewer feels you didn't answer their question they reject you.

So it looks like all the top paying places interview this way.

I guess part of what they're paying for is your tolerance of this BS then. Only you can decide if you think it's worth it.

Hard agree there. I'm quick to say "I don't know (yet)" because I don't want to waste everyone's time while I stumble through a bunch of made-up answers trying to sound smart. If I were punished for admitting I didn't know the details of something, I'd leave in a heartbeat.