It's like a dance. Your are supposed to lead deriving the solution, interviewer is supposed to follow closely and judge your performance.
If you expect to ace an interview just like that then you also need to be lucky not accidentally getting asked to stuff, which you always need to Google an answer to, basic algorithms and patterns, which you usually kind of know but could slow you down during the precious 40 minutes you have to show off your skills.
And lastly, people do prepare for these. If you do not do that, you might simply make a slightly worse impression than someone who did.
That being said, I mostly did not prepare for the coding interviews. I focused on the design and behavioral interviews.
