Because the Right mouse button is on the right and the left mouse button is on the left...
To me Right and Left are learned.  When I was young when i needed to distinguish right from left I remember picturing an Atari Console which had right and left players marked clearly.  It is more rote now than it used to be but it still does not always feel natural.  I sometimes get momentarily confused if someone gives me a direction like turn right if I am expecting to turn left.  Other times I do not distingish the difference and turn left instead.  If I have no expectation there are still times where I need to think about which direction that is.  I actually know rmb and lmb not because of right and left but because it is in my muscle memory.  I know left and right mouse buttons not because of the right and left directions but because that is how i have thought about them for nearly 20 years.

So basically left and right are nothing more than common labels.  Much like drivers and passenger side.  But I like to think of it as you are either right or wrong handed.  And to be nice instead of saying wrong handed we said left... so going forward just replace left with wrong and it will be much easier.