It's probably less expensive than field setups in large part due to siting near existing infrastructure. And it doesn't have to out compete residential, it just has to be a net positive investment on its own terms, out competing an otherwise unshaded parking lot that isn't leveraging it's airspace for anything.

Rather than a tax on lots it's something that turns them into a source of revenue generation.

In places with a lot of flat empty land, solar farms are a lot cheaper. South Korea doesn't have any flat empty land though..