It is clear that the idea of a jam is to have fun, but where is the idea of waiting for the jam to begin and of its theme if in the end anything can be uploaded, if we ignore that fact it does not make sense for a jam to launch a specific topic to follow.
for some reason you have to wait the day of the start of the jam, to see the theme and thus create the game from the time limit assigned along with the theme, ignoring this, it would be unfair, as I said anyone would upload a game already created since a lot and it would not be fair for those of us who are following the subject. Precisely for this reason they never give a specific topic, to set limits and test the skills of programmers during that limit.