1. You're implying that increased housing density makes it harder to find infrastructure.
This is just false since high density makes infrastructure cheaper per person.
2. Why should it be the developers jobs to build out new infrastructure that isn't directly needed by their building. It's their job to build housing which brings people, those people bring taxes and those taxes pay for upgrades to infrastructure.
I'm not implying it's harder to find infrastructure. I'm arguing that developers don't want to pay for any infrastructure improvements needed to keep services operating in the same manner with more people. Like everyone else, they're self interested and will act in ways that maximize their own interests, even if it externalizes certain costs to the surrounding community. I suspect it's also more expensive per person to upgrade existing infrastructure compared to building it new.
In terms of who should improve infrastructure, it doesn't really matter who does it, but I think either local government or the developer should be required to do it and not be allowed to let things slide. I focus on the developer because they're the first in the process, but it doesn't matter if instead the city requires the developer to build in the infrastructure improvement costs and does it themselves. The only position I have is that it should get done.
I gotcha. I'm also thinking of mass transit, parking, retail, and probably a few more things I've forgotten about. Even the local geography/weather can play a part in terms of pollution. If we go from a subdivision with 40 houses to apartments with 4,000 units and update the existing infrastructure, 4,000+ cars starting up every day could result in a significant increase in exhaust pollution that we may not be able to design around. The climate/geography are going to be the climate/geography.
2. Why should it be the developers jobs to build out new infrastructure that isn't directly needed by their building. It's their job to build housing which brings people, those people bring taxes and those taxes pay for upgrades to infrastructure.