Why interviewer asks java 8 stream questions to solve in notepad and they want that we can memorize each and every function of java 8 stream API. Like I failed

Why interviewer asks java 8 stream questions to solve in notepad and they want that we can memorize each and every function of java 8 stream API. Like I failed 2 interviews just because of this and I was able to solve those problem with normal flow of code but they wanted in stream API. 
Does anyone really remembers those functions or what ?

Java interview process