Asklemmy

Because people prefer living there and (in the US) because low density development is given legislative preference

The thing I've heard is, think of how when you're a mile away from each neighbor, it's your tax dollars paying for the road, sewer, sidewalks, water, electric, gas lines, for a half mile in each direction. Initially and for maintenance and replacements. That's why a lot of rural areas just don't have sidewalks or fiber internet or sometimes they're using well water.

In a city duplex, you're paying half the utilities for like 20 feet in front of your house.

It just is more efficient to live closer together, the reason cost of living goes up is because everyone wants to live in the city and employers want that supply of workers so they try to get in or close to the city too and it's a virtuous cycle of concentration. But housing supply being what it is, and all the jobs being nearby, means housing prices go up. Still worth it to most people hence why there's still demand, but higher than living in a place with fewer jobs and amenities.

Subsidies. Both in form of roads and home ownership incentives being focused on single family homes. The fact that renting is the primary way to live in the city seems detrimental to it being cost effective too.