It's because most activity takes place along, or very close to, the horizon line - which is wide, and not tall.
Look out your front window - chances are the street extends toward your left and toward your right. A car driving by does so from right to left, or vice versa. Kids playing do so on a plane even with the ground, more or less horizontal in your your line of sight.
Now look down - you'll see the ground. Nothing very interesting there, at least not comparison.
Look up - sky. And it all looks more or less the same. Nothing really going on there, either.
So our eyes, and our brains, are used to seeing and processing the world around us in "widescreen" because that's where pretty much everything is.