Is there any reason why most of these 3d engines use one-letter variable names?
Because 'x' is easier to type than "getHorizontalScreenSpacePixelPositionRelativeToBottomLeftCorner()" ?
While true, you end up reading code with your variables more often than you write your variables so I tend to optimize for readability over ease of typing.
In a setting with much arithmetic "x" is also vastly easier to read. See pretty much any mathematics or physics paper or textbook -- made for reading...