From a carrier's standpoint, great coverage comes at various technological expenses and certain restrictions that are out of their control. For instance, each provider is limited by the FCC to a number of frequencies it can use in any given city.
Given a limited number of frequencies available in the spectrum, in order to sustain the capacity needed for urban coverage, frequency reuse is required.
To put it into an extreme example, if there was only one tower covering all of New York City, only a limited amount of simultaneous users could use that tower at once due to FCC regulations, say 100 frequency slots. Thus, 100 users at once could use it.
Conversely, if each house had its own tower, 100 users would be able to talk on their cell phones simultaneously in each house. Since each tower now only has a few feet to cover, power consumption is greatly lowered. Once a guest walks to a neighbor's apartment, the current tower would hand off the user to the next tower, freeing up a spot of another person.