Because its hard to find existing construction that’s

1. In the right location.

2. Dense enough.

3. Cheap enough.

4. In good condition and zoned properly.

The last bullet point is the real kicker. It’s very tempting to point at older real estate, often shuttered light industry or commercial, and ask about converting it. Unfortunately a lot of these buildings are illegal/hard to rezone as residential, or have an uncertain cost to convert them for habitation. And of course the financiers of any project hate uncertainty.