Having a fairly small, well defined problem, working in a small self-selected team, with no outside interference and typically no clients outside the team. No managers, no PMs, no bug reports. This is not how day-to-day projects get done.

Regardless of all of that, in my experience a typical impressive hackathon project is still just a barely working demo that benefitted from a significant amount of research and planning beforehand, and will require an even greater amount of hardening and polish afterwards.

There is no magic, it's just a vastly different kind of work environment with both inputs and outputs incomparable to day-to-day work.