The treadmill keeps you safe and in place. Go outside where the path actually goes somewhere.

Controlled conditions are comfortable partly because nothing surprising occurs in them. Same speed, same view, no hills you did not choose, and also no arriving anywhere. There is nothing wrong with the machine, but if every part of your week runs at a fixed pace, you get fit at doing exactly what you already do.

Add one unpredictable thing. A route you have not run, a pickup game, a class where you might be bad at first. Being slightly out of your depth is the input that the controlled version cannot give you.