I recently read about safe mud window - it states that mud pressure in wellbore should be high enough to prevent well collapse, and should not exceed formation fracture pressure. But consider deep mine tunnels - there is no "mud pressure" at all, just air. Mines depth could exceed 3km and tunnels withstand this overburden rock mass. So why wellbore collapses in lack of pressure, but deep mines does not?

I recently read about safe mud window - it states that the mud pressure in a wellbore should be high enough to prevent well collapse, and should not exceed the formation's fracture pressure. But consider deep mine tunnels - there is no "mud pressure" at all, just air. A mine's depth could exceed 3km, and its tunnels withstand this overburden rock mass. So why do wellbores collapse with a lack of pressure, but deep mines do not?