I would think that the vacant lots are the ones freeloading, keeping people from living in, working in, or otherwise being able to access an extremely high valu

I would think that the vacant lots are the ones freeloading, keeping people from living in, working in, or otherwise being able to access an extremely high value area. The expenses of running roads and sewer are completely dwarfed by the gain in economic activity. Dense, high land value areas are the tax bases that subsidize the far less utilized and more-expensive-per-connection sprawl.