I think what makes this possible is simply the scope of things. When we make games for a game jam, we tend to be much better at scoping our games. Doing that for our dream game is quite a lot different, because scope creep becomes an issue. Also we tend to think we can do way more than we actually can. The bigger the project, the harder the risk of falling into the trap of getting tired of it. And that is strange as we loved it of course. But think of it as a favorite item you have. How long does it take for another item to take this favorite spot? Usually not very long and the thing is, this is also true for game development. We love something until the next nice idea pops up. Game jams you can't just change to the next best thing and there is no time to overthink or add to the scope.