They are shit to live in.
Cheap to make, quick to build, people will buy them because they are brand new and look nice in the brochure.
Then the cheque clears and they realise that theres no storage, the rooms are all tiny and those apparent quality finishes are actually that great.
Build each house for £50k. sell each house for £150k. 100 house development nets you 10million profit
You think developers get rich by 'spending' money?