In Italy 13 isn't actually considered such an unlucky number; it's considered bad luck to have 13 people at the table, but otherwise it's often even considered a lucky number. On the other hand the most unlucky number for an Italian is 17, supposedly because the Roman XVII was an anagram for VIXI (I lived, i.e. I'm dead), and venerdì 17 is the most unlucky day.
Really! Cool! I didn't know that. I figure the table thing relates to the Last Supper, and then XVII = VIXI makes as much sense as the theories for the unlucky qualities of 13. The Chinese have a better reason to think 4 is unlucky: It sounds just like death (tones difference). I laughed when I learned that 666 is a lucky number for the Chinese and Japanese!
True, 666 is lucky here in China.
In upper-class hotels in China usually the 13th floor is "missing" and the 4th floor is "missing" too.
Sometimes there is a floor 12b or 3b. But when you walk the fire escape often you see signs "4th floor" or "13th floor". The disguise is only used for the numbering of rooms.
I translated the Italian sentence to "Why not number 13" and it was not accepted by the Duolingo system. Writing numbers as numbers instead of words has been accepted by Duolingo in every other question I answered.
Maybe the article "the" is strictly necessary in this sentence?
I assume your question is why doesn't Duo permit the number itself to be written. In this context it's mostly important that you understand the word itself, but it's not actually proper English to use a number in this kind of context. Numbers in writing are reserved for things like math equations, prices, times and dates, and things like that.