You can really add things like sewers after the fact. You either need to add them before or during the addition of more (higher density) housing. Generally, this means that the builder of the new housing is required to foot the bill for that added capacity, which can make it not worth the cost.

The same tends to be true of roads, but with less catastrophic results. In a town where going 2 miles can take 20 minutes, adding a lot more housing without adding road capacity is not a great plan.