because its the boss of a stage/level/area or chapter you are trying to beat to advance to the next part of the game. -_-
Free Step Dodge
Crazy_Chin_Guy
Because you’re fighting a boss, jesus christ man.
Bacaww
Basically youre fighting a boss 
Lulu
LoL... yeah that doesn't really explain it.
Brute
"Why is fried chicken called fried chicken?"
"Because it's a chicken that has been fried."
"LoL.... yeah that doesn't really explain it."
"Because it's a chicken that has been fried."
"LoL.... yeah that doesn't really explain it."
Brute
Should I pretend that you speak English?
Lulu
LoL... What do you think ?
Brute
Probably not.
Lulu
LoL... yeah thats what I thought.
Bacaww
Do you expect a 12 page long answer which includes scientific research about why a boss fight is called a boss fight?
So really.... why is it called a Boss Fight ?