It is compensation for all the sugar, caffeine and UPF causing dehydration IMO. And aircon perhaps.

People in Spain, Portugal, Italy etc don't carry 2 gallon water bottles everywhere.

The ones who have better diets are just going along with the crowd (though not necessarilyin better shape), using the oversized pacifier to signal they're healthy ("people who exercise have water bottles. Therefore I must show off a water bottle")

And kids are products of the parents