Thats all of the point of a game jam. Or else people will just upload their premade projects which they must have worked on for months or even maybe years lol:D
Then don't join a game jam...? The whole point of a jam is to make something small in the given timeframe. If you can't finish in time, pick longer running jams (there's one that ends in 2030 right now) or simply don't participate. It kinda seems like you just don't like the idea of game jams to begin with.