Water and electricity and gas are generally less cost-efficient to provide to less-dense neighborhoods, as the length of wire & pipe (and maintenance overhead) needed to serve a given number of residences is significantly higher. This is likewise why electricity and phone service were not provided to rural areas until subsidized by the government by taxing urban dwellers.
To new neighborhoods, you're right. To existing neighborhoods, you're wrong: the infrastructure is already there. There's no reason to raise rates for people for something that's already paid for. That's like adding a toll to a road that's been there for 50 years; it's just profiteering.
Maintenance is not going to be any higher for lower density; it's not like you see electric utility workers in the suburbs constantly, repairing stuff. And access is more difficult in higher-density housing too: in a suburb, you just drive the truck up to the transformer, but in a large building, it isn't that easy.
As for rural areas, there's a huge difference between urban and rural areas (where houses are miles apart), and urban and suburban areas (where houses are 20 feet apart).
