There are two vague reasons for this (actually depends on phone architecture)
1) Most of the smartphones are built with Lithium Ion battery. by virtue of its chemical ingredients , even if not used it looses it’s capacity from 1.5-3 %(for a month approx) due to chemical reactions .
2)when you switch on your phone again by pressing the button the software gets loaded to RAM and etc.. stuff will be done so as to bring phone to usable state.
now that means when someone presses the ‘ turn on ‘ button there must be someone who detects that event. so even when phone is switched off there will be some power usage for detecting this event
so hence these are two major reasons