20th floor apartment

A man living on the 20th floor in an apartment building got up early each morning to go to work in a downtown store. He always went into the elevator on the 20th floor and rode down to the entrance (1st floor). When he came home he always rode the elevator from the entrance and up to the 9th floor. He walked out of the elevator and walked the stairs the rest of the way up to his apartment on the 20th floor.

Why desn’t he take the elevator all the way up to his apartment?