The problem is that it is not about the amount of water delivered, but the capacity to deliver water.
You might be able to fill a swimming pool using a garden hose, but it will take days to do so. You probably would prefer to pay for a sufficient supply to be able to fill it in hours rather than days - same amount of water, totally different demands on infrastructure.
Bandwidth is the requirement for the end user. Bits used is of concern to the provider for their transit cost, together with bandwidth capacity to meet the peaks of demand.
Bandwidth is a fixed cost, transit cost is a variable one.