There's a variety of reasons why people look at the ground.
For me, for one, I got cats, I don't wanna trip over them. Especially not with my kid. I tend to bump my feet a lot on my kid's toys and other stuff or objects lying on the ground (even permanently or slightly moved). And I need to be careful when I walk downstairs because I am clumsy. I hate, absolutely hate, stepping into dog shit when I'm out and about. If I'd step on a slug, that'd be disgusting but dog shit is a whole different level. And its rampant here. Finally, many 'normal' people use smartphones whilst on the move (I personally generally don't). That being said, I do try to look straight, I try to look people in the eye, but that's secondary to the above examples. Which are IMO related to survival and conveniency.
As for your linked Wikipedia article, quoting from it:
"However, one of the main assumptions of the Gokhale Method, that people in less industrialized societies have less back pain,[4] is incorrect: mainstream science states back pain occurs to a similar extent in all cultures.[20][21] As of June 2015, the Gokhale method had not yet been scientifically studied."
Furthermore I encourage the reader to read up on the Feldenkrais method. This Wikipedia entry shows how controversial it is (Gokhale method seems to be based on that).
[1] https://en.wikipedia.org/wiki/Feldenkrais_Method