We can't really change it because there are too many plugins and behaviors (including third party ones which we don't have control over and can't change), tutorials and manual entries which already use this method.
FWIW, it makes sense mathematically, because the formula for movement at an angle is:
dx = cos(angle) * speed
dy = sin(angle) * speed
cos(0) = 1 and sin(0) = 0, therefore angle 0 is moving to the right.