Ok, everyone else already took the good answers.

Flights receive routing instructions and for the most part are generally routed via the most direct path. On UA (channel 9) you can hear the pilots ask for direct routes to the different markers if they have been given a less direct route.

Sometimes the route is changed to a less direct route due to weather or flow control. For example, I once was on a BWI-ORD flight that took 3.5 hours. The reason:severe weather in the normal flight path meant we took the southern route.