American buildings are often multi-storey, meaning that high-pressure hydrants (fed by a very large water pipes) are needed in order to ensure that hose jets can reach the tops of the buildings. So they need a dedicated network of high-pressure pipes, leading to some very big valves (which are needed to withstand the pressure and aren't easily positioned below the ground). Our buildings aren't usually as high, so hydrants can simply be an add-on to normal domestic water supplies.