I just happen to ride a lot of above-ground trains. E.g. much of the New York City subway goes above ground, sometimes pretty high, in Brooklyn and Queens. Same applies to quite a few cities (e.g. London and Moscow, off the top of my head).
But okay, let's imagine out underground walkway. Since it's underground, you can't walk on and off where you want; you only walk off where there's way up to the ground. This eliminates much of the allure of the system. Now you want to plan ahead and be ready to get off when you're next to the exit. If you missed your exit, you have to move to the next exit, or maybe cross the tunnel and go to the band of the opposite direction, and walk back a bit.
Please notice how the low-speed bands then need to run the whole length of the tunnel, adding little to the carrying capacity: few people want to travel at the speed of walking when traveling at the speed of a train is a few meters across.
If we let only the faster band(s) through the length of the tunnel, and limit the slower bands to the exit areas (pretty long and wide), we'll face the problem of walls: our band is moving fast, and nothing separates it from the walls. So we sort of need inner walls on the band. But they need doors to allow people in and out in the exit areas. And the doors need to be shut when the band is in the tunnel. It starts to look like a... train? Only an endless train with a peculiar way of boarding.
This endless train also has a downside: if anything goes wrong, the whole band/train has to stop, it cannot be re-routed. What if we segment the band into individually routable parts? Only then they'll have to stop at exit areas to allow safe getting on / off them.
Well, we have reinvented the subway.