Why do these two lines not look perpendicular in Tikz/PGFPlots?

This creates a simple plot with axes and the lines y = x and y = – x.

documentclass{standalone}

usepackage{tikz}
usepackage{pgfplots}
pgfplotsset{compat=1.13}

begin{document}

begin{tikzpicture}
begin{axis}[
    axis lines=middle,
    ticks=none,
]
    addplot[thick, black] {x};
    addplot[thick, black] {-1 * x};

end{axis}
end{tikzpicture}

end{document}

Produces this (the border is an artifact of my screenshot, but its not important):

If you hold up a 90 degree angle (say a piece of paper) to the intersection of the x and y axes you can see that they are nicely perpendicular.

However, if you do the same for the lines y = x and y = -x, you see that they are not perpendicular.

What gives?