For one thing, it is more expensive per volume than gasoline. It’s a great profit maker: you go to a location that may already be desperately short of water (a quarter of the world’s population lives with water stress), pump it up for a fraction of a cent per litre, put it in throw-away bottles that now everyone knows is murder, and sell it for a huge profit.
Or of course you can just put tap water in the bottle, and tell a lie on the label.
Buying these things is hugely unnecessary in countries with universal sanitation. But Katherine Martinko has shown that you can avoid bottled water even in a third-world country.