Soda is highly subsidized in the US, it is a consequence of US gov subsidizing corn syrup, so on a ratio of wages to prize sodas are very, very, very cheap, and in a country with few public bathrooms or public fountains, the easiest way to have a drink while on the go used to be sodas
These days it is more common to ask for water, but even then, that will be bottled water and the prices can be similar to soda
Are we talking about the same US? I don't live there anymore, but I'm always surprised by the high number of indoor fountains in publicly-accessible buildings (as well as many parks and outdoor locations) there. For instance, every library, shopping mall, or larger store seems to have one near the bathrooms.
Meanwhile, in some European countries, for instance, outdoor fountains can be more plentiful which is fantastic in its own way, and indoor options are much less common. In others, neither outdoor nor indoor options exist so one is forced to find a public bathroom to use the faucet.
In some other countries, both public bathrooms and water fountains can be very rare, which unfortunately leaves no viable option.
Soda isn't really subsidized. Yes corn is subsidized but the main cost of soda is advertising, packaging and distribution, the flavor and syrup (or in the case the sweetener) costs nothing.
These days it is more common to ask for water, but even then, that will be bottled water and the prices can be similar to soda