The reason our line looks like a square is because we are looking at it directly from the players perspective and the line is being continuously updated every frame which ensures we are always getting the same perspective as we look around.
Real life example
Find something long and straight like a pencil and hold it up to one eye staring directly at the centre of the small end. close your other eye. You will get the exact same perspective as you do in the game and hopefully, understand why we see a square.
PS. use some common sense if you try this, I will not be held responsible if anyone loses an eye