Yeah, it's that magical, cargo cult thinking again. Association with 'health' and also the idea that if something is a good thing, insane amounts of the same thing are even better. It's like folks who chug megadoses of vitamins; mostly they end up with very expensive piss, but occasionally someone damages themselves by going OTT with something the body cannot expel.
As for the 'why' ... It has become part of the cultural received wisdom in the past ten-twenty years - and obviously not at all anything to do with the marketing of various massively profitable and expensive (compared to tap water) bottled waters and sports drinks during that time - that we are all mysteriously dehydrated and said dehydration is why we are (fill in what ails you).
Nuggets of entirely scientifically-unsupported bullshit such as 'if you feel thirsty. you're already (cue doom klaxxon) DEHYDRATED!' have been repeated so often in various media that people are convinced if they're not sipping water every ten minutes, clutching a bottle of liquid to them whenever they step out of the house or chugging vast amounts of water during exercise, they're putting their health and possibly very lives in danger.
The fear of dehydration has become so bad that water intoxication (yes this is a thing) has become a relatively common risk factor to marathon runners as the more amateur end of things puts so much water away during an event in fear of becoming dehydrated that they mess up their electorlyte levels and end up in the hospital. Occasionally someone dies. Every so often someone goes balls to the wall on a diet that requires the absurdist amounts of water you mention and
also dies.
