If everyone charges $20 for a burger, people will still go to McDonalds. In such a state, no individual place would dare charge $10 again. Why cut revenue to nearly half? It’s not like they have the capacity to sell 20x the usual volume…
I’m not convinced that basic economics works when output capacity isn’t fully scalable.
Same reason no lawn care team tries to deeply undercut competitors - they can only mow so many! (Barring that they hire more helpers)