@Doctor_Malsyn
Probably because it's historical and cultural (see "kabuki")
In school manga, it usually has been always the theatre/drama club who features crossdressers too usually with a female club member playing as the "prince". not sure how that all started - maybe there was a live action/drama movie that did it first, or it was already a common thing in real schools for when kids are trying to do something unique (school setting manga originally were written with a nostalgic vibe)
The fetishism is a whole another topic, and is probably related to the modern otokonoko phenomenon (aka traps)