"All of us are confused tho on WHY 1 month long?"
1 month long jams are far from unheard of. As ZackGriest stated, if you look at the Itch.io jam calendar, a large amount are exactly one month long. For example, the PowerPoint Game Jam started on the first and ends on the 31st. There are tons that are even longer than that.
Buildbox can make their jam however long they want.