Hot water continuously radiates it's heat into the environment around it. City wide hot water infrastructure would be hugely inefficient and impractical as that water would constantly have to circulate to and from homes to be re-heated and re-circulated.

Literally heating massive quantities of water; just to pump it out into essentially a field (of pipes), wait for it to cool, then pump it back and do it again.

Without that recirculation to keep hotwater immediately available at each home/tap; you'd be waiting hours for all the cooled off water to flush out of the pipes and be replaced with hot water, wasting all that water while you wait. (kinda like waiting on a typical hotwater tank, but x100)

Electric hot water on demand combined with green sources of electricity should be the goal.