The point is that time spent developing rushes by, and 6-12 hours is the absolute minimum to have something worth judging. (See the 7 hour warmups for the 7DRL/7DFPS competitions)
Last LD it took me more than an hour just to come up with an idea. It took then another ~8-12 hours to get everything working well enough to start hacking together a storyline.
Now of course given the right tools, I could make a decent game in a day (using my custom engine which has undergone 5 or 6 iterations and can really only make one or two types of game). But it would hardly be worth the bandwidth taken to download it.
If you hang around on Reddit’s /r/gamedev I’m sure you’ll find some sort of gamejam to participate in.