FOV = Field of View
Because it’s mapping a field of view (which is a cone) onto a flat rectangle. If your monitor was shaped like a portion of the outside of a sphere, then we could do the mapping differently and it wouldn’t be distorted, but only for the FOV that matched the size and curvature of your monitor.
Think about the screen as if it was a window. When you look through the window you can see a certain angle of view, measured as a portion of 360 degrees around you. The larger the window at a given distance the greater portion of your view it takes up. In concept an infinitely large window would approach 180 degrees of your view.
The game doesn’t know how large your screen is or how far away you are, so you can adjust the field of view it displays to match what you would see if the screen was a window. But you can also adjust it to whatever you like, and compressing a larger field of view into a screen that takes up less of your field of view results in distortion.