They get away with drip feeding because there’s no competition, because it’s really hard to get permission.
A simpler planning system would allow smaller developers to operate and they would buy, build and sell as fast as they can.
The reason big developers drip feed is that they have 100 identical houses in the pipeline so they don’t want the price to drop at all, because it affects the price of the houses in the pipeline.
The small developer sells quickly so that the cash is available for their next project.