Looks like its this song one by three dog night http://www.threedognight.com/l_one.html
The only reason is that you happen to speak English. If you grew up speaking some of the other languages that are popular around our globe, you'd call it something else. Here are a few examples....
anything times one is itself example: 1x1=1 1x2=2 1x3=3 1x4=4 and so on.
An odd number is one that cannot be divided by 2 exactly therefore 1 is an odd number
Because. That is how the English language has worked for years and years and years and makes the most sense.