Here, the gas company (contractor of same, really) put in detection wires so that the location of the new, plastic pipes can be sensed. However, the contractor ran them in the most convenient fashion for them, meaning often on angles from a single starting point, and also rather close to the surface in places. Their are minimum depths they are supposed to maintain at various positions from the main to the meter. I'm not sure they always did so. Some homeowners who landscape or garden more aggressively, may be in for a surprise.
Going deeper is possible but does have drawbacks; access shafts must be deeper and it takes a longer time for people to move between platform and street level. New York's older cut-and-cover sections also do not deal with ventilation systems, because regularly installed grates to the surface essentially use the piston effect to circulate air in and out of the system.
Quite a few of London's lines are deep, but they generally have escalators which I don't see a lot of in Manhattan.
When you dig, you have to deal with all of this. If you break something, you have to fix it -- and in the case of fiber or multi-hundred-pair copper, you can't necessarily just "splice" it or easily reroute around your construction site.