Because you asked them question out of syllabus. Was the person informed that the interview wouldn't involve Leetcode?
The person spent months prepping Leetcode just to get into BigTech and then you come out of woodwork with your "special" interviewing material. Imagine prepping for a calculus test for months and then the teacher comes out and takes a "simple" test on trignometry. Would you be able to do it? Not only that, all his/her friends got a normal calculus test which they passed with flying colors.