Economics plays a big role here, In summer a normal person needs almost 5/6 lt water in a day so 15x6=90 and if journey lasts more than 20 hrs the consumption increases.
Suppose you are 2 person traveling so may be the food/water/snacks bill can be around 450/500 rs for single passenger.
It is very tough to find chilled/filtered water even at major stations and rarely 2lt water bottles are sold in IR leave 5lt can.