I invert the Y-axis in video games because I think that it is the most accurate method of controlling a character with the mouse that is practical. My reasoning behind this is that when you look up in real life, you tilt your head back, and vice versa for looking down. Another way to view this is imagining that you have your hand on top of somebody's head, pull your hand back to make them look up, push your hand forwards to make them look down. This analogy doesn't really work for the case of X-axis movement however because moving your hand left/right would tilt the head left/right, which is known as leaning in video games, and leaning with the mouse is impractical and not much better than just using a keyboard for it whereas the functional difference between aiming with a mouse versus aiming with a keyboard is huge.
However, the analogy of you moving your own head back to look up/forwards to look down does not apply to the inversion of the X-axis because when you look left in the real world, you turn your head left, and when you look right, you turn your head right.The shown image is a comic that I found the hand-on-head analogy from. (Click on it to see the image in full screen).