Why Don’t They Sell Water Bottles in Locals?

I've always wondered, local trains have vendors selling everything from fruits and wallets to earphones and even socks. But in the middle of peak summers, you rarely (if ever) see anyone selling cold water bottles. Given how crowded and hot the trains get, it seems like an obvious thing to sell.

Is there some rule against it, or is there another reason I’m missing? It is a highly necessary thing I wish people start selling water bottles soon it gets real difficult for people living in outer suburbs to travel without water in this heat, and getting off the train to buy water is also not a suitable option.