why the right hand side ????

hi guys, my question is why do 99% of bike exhaust / mufflers exit out the right hand side of the bike??
my bike has a 4into2 system that is suppose to be one piece but one of the previous owners have had the headers modified with a flange before it splits into 2 so that it can run a 4into1 system.
the easiest way of doing this is to run a muffler out the left hand side as it means less bends and retains easy access to the oil filter.
do anyone make a muffler for the left hand side of a bike as i don't really wanna buy 2 mufflers for a twin system and only use 1.
thanks.
my bike has a 4into2 system that is suppose to be one piece but one of the previous owners have had the headers modified with a flange before it splits into 2 so that it can run a 4into1 system.
the easiest way of doing this is to run a muffler out the left hand side as it means less bends and retains easy access to the oil filter.
do anyone make a muffler for the left hand side of a bike as i don't really wanna buy 2 mufflers for a twin system and only use 1.
thanks.