Which of these "costs" do not exist in suburbs? (except subway, the analog of which is highways to the suburbs). All of those are present in suburbs too.
Beside, you've already got your answer. Read the last sentence: This is a scalability issue. And my understanding is that it's difficult and expensive to build these things to scale to urban densities.
Density is the issue.
I'd submit that the experience with broadband/cell phone costs+coverage in the US vs. other countries shows that it's cheaper to provide services if population density is higher.
Now, you may end up having no higher a ratio of towers to subscribers than in the suburbs. But what you do have, is a lot higher costs to rent an easement for the tower. Renting antenna space from a building is a lot more expensive than an easement in some back yard.