well.. it is not exactly the case, but in a way, it is . every page has actually "2" names , one is the name you give and one is the ID, which is the number. If you will give a "name" which is a "number" - it might override another entity (Can be a post, page , attachment, or even a draft) with the same ID. that would result in multiple errors and 404 , depending on your permalink structure.
SO , it is not such a good idea to do that, but if you really want to go with it first try to change your permalink structure and if that does not work, you will need to hook the sanitize_title() - but I will still advise to avoid such a practice.
( read here for more:
- it is an old post from previous wordpress releases - , but maybe will let you understand WHY it is doing what it does.)