Because it depends on the rules of each Jam. Some have no problem if you enter with a game already programmed or that you have half programmed.

The concept will be because it is better to give freedom.

If you have a game that can get into 2 or 10 JAMs and doesn't break any rules, there's nothing wrong.

The problem is abuse, but any system you implement can always be abused, for example if you program that a game can only enter one JAM, nothing prevents you from creating 10 pages, uploading the same game with a slight change and uploading each page to a different JAM and would be more difficult to detect than with the current model.