[sandworm101]

>> adding things in an area that's already served by infrastructure

Unlike in video games, "adding" load to infrastructure can cost more than running new lines. Everything from pipes to power lines have finite capacities. If a city sewer system is at capacity, as many are, dropping some more people into the middle (replacing a parking lot with houses) will require possibly ripping up the old sewer/water/power lines to expand them. And expanding their up/downstream connections. That can impact far far more than the local connection, often costing much more than green-field development. Imaging how much cost to open up and expand a sewer under any Manhattan street. Compare that to digging a trench through a green field out on the outskirts of town.

[ProfessorLayton]

Everything from pipes to power lines also have a finite lifespan, and many cities are succumbing to crumbling infrastructure as the low density suburban tax base isn't covering their costs, and is unsustainable in the long term.

Yes it costs a lot in absolute terms to rip up and expand a sewer lines in Manhattan, but that doesn't mean it costs more per person being served by that infrastructure.