So do hot peppers.

You don't have to propel yourself forward you just have to move your legs forward.

Very dead experience for your brain too I think.
Also uneven ground (especially barefoot or in something like Nike Frees) has HUGE benefits and in my opinion is by far the first option running if possible.

Outside has many benefits also - sun (extremely underrated - even if cloud cover), hopefully vegetation (nature) but being outside, even in city, is closer to nature than inside.

Just as one example the sun - vit D is a small portion of it. It is not only a huge key to life but quality and health of life which absolutely affects performance in my opinion.

fitness and health are not synonymous

health creates fitness but many fit people are not healthy (mare athletes today die than in the past)

your body, propriocepters, brain, sense organs of all kinds are mostly unused and bored (use it or loose it) when being on a hamster wheel (treadmill). You are working your body systems at a fraction of their potential in my opinion and experience.

(sun is bad for you is laughable in my opinion) - if you get outside more you will be healthier - healthier leads to increased performance