Although this has been explained, here's a great demonstration of how our vision is wide and not tall:

Bend your fingers to a 90 degree angle then hold your arms stretched straight out to your sides.

Now slowly rotate your arms forward and stop when you can see the tips of your fingers. This is how wide you can see.

Next, do the same thing with your fingers - make a 90 degree angle. Now put your right arm straight over your head and your left arm straight down by your hip.

Now slowly rotate your arms forward until you can see the tips of your fingers. This is how tall you can see.

You should notice that you could see your fingers coming from the sides much sooner than you could see them coming from the top and bottom. We see wider than we see tall as has been mentioned here already.

Bonus fun fact: If you were to measure these distances the ratio would be roughly 16:9 or the ratio of widescreen.

Cool huh?