Or y'know...maybe homeowners don't want to have their neighborhood become overcrowded. Not everyone wants to live in a megacity with traffic, air pollution, constant bustle, and restricted personal liberty.
If financial gain is your only motive as a homeowner, you would actually WANT an increase in density. As you cram more people into a smaller space, the land underneath becomes more valuable. Increased density attracts amenities, restaurants, businesses, nightlife...etc, all of which raise the value of your property.
Homeowners care about things other than net worth.