Pull up circuit current flows?
Because when the current gets to the "crossroad", it has two options to go to GND. One is through R2 and the other is direclty to GND. In other words this is like putting R2 in paralell with a resistor whose value is zero ohms. This lead us to a zero ohms equivalent, which is like a short circuit (left path).

In a intuitive way, it finds no resistance to go to the left while there is R2 if it goes to the right. The current division in a "crossroad" is a proportional division calculated by the resistance ratio of each path. Since one path is free, all current goes to there.