Hi stuff...: It is because of an old superstition about the number 13 being unlucky. If you go to China they don't have a 4th floor because the word for 4 and Death are the same (or something like that) So basically it is because people are silly ;)
In the real world if a building of any type has more than 12 floors,do the math ,there is a 13th floor. Its truly amazing to me how the simple suggestion that floor 13 is now 14 since we say so,actually works.Here is another suggestion, anyone who wishes to have the spell of the 13th floor removed, send me money.I will insure that you believe you are safe as long as we call it FLOOR 14. NLP .