No commentary about entertainment venues removing/hiding the water fountains and making carry ins (like my refillable bottle) illegal for security theater reasons? I'm not saying that explains 35 gallons a year unless you go out a heck of a lot more than I do, but it is something of a gateway drug, that once you get used to going to the state fair and spending $7 for a brat and ... $2 for the bottled water to go with…
Of course, it also seems silly to pack a large empty container.
I'm more likely to use a disposable bottle if I'm going to an event, or I'm walking around a city all day as a tourist, and want to be able to ditch the bottle rather than carrying it around empty.
That said, I virtually never buy water, I dislike it on principle and only do so in special circumstances.