If you use more water, even if there is plenty of delivery capacity, they could literally run out of it. There is only so much in the reservoir.
If you use more electricity, even if there is plenty of transmission capacity, they have to burn more fuel to generate it.
Transferring more bits doesn't risk depleting the supply of bits and doesn't require burning more fuel. The worst it can do is consume all of the available transmission capacity. But the amortized cost of a bit is very low -- if you charged true cost then it wouldn't meaningfully deter usage, so you'd still need about the same total amount of transmission capacity. At which point charging for usage serves no legitimate purpose.