This is some stuck up shit right here. People are getting out and exercising and we're judging them for not doing it how we like. Gyms are much more than treadmills, treadmills are easier and more consistent for people starting out, the summer heat is brutal and gyms are air conditioned, or maybe you just wanna watch something to take your mind off the run itself. There's lots of great reasons to use a treadmill at the gym.
An additional point to treadmills is that it's easier on my knees and ankles. Also, I know we're not supposed to talk to people at the gym but I chat --ever so briefly-- with the regulars that go at my times. It's nice to say hi to a familiar face every now and again. As I'm getting older and do remote work, it's useful for me.