I think @Alex Franks and @Jay Hinrichs pretty much nailed itm, but I will expand just a bit and offer a few different perspectives.
It is a different game and really a different industry. To build ground up you are a General Contractor or are hiring a GC. To rehab/flip that isnt necessary.
The other thing as alluded to is risk. If you buy a $30k house, its hard to do something to that house to render it worthless. You might make it worth $20k mid rehab but it still has intrinsic value to a large segment of the home buying (both retail and inner industry) world. A new construction project where the builder goes belly up half way through, watch those sit for years. No one wants to touch it. Yet you have the same $30k invested and seemingly no value if you cant complete.
To me green field projects present the most repeatable profits, albeit at a smaller gross margin per door. The challenge I have seen over and over is new home builders always scale beyond the end of their financial boots, and when the pull back happens. And it always happens. Most new home builders lose it all. Then they are back swinging big lumber in a few years on the next cycle.
I really need to get up with Alex though and we have been talking abut it for a few months because build to rent is an interesting concept to me. The final piece of the puzzle is if I save up $100k and want to turn it into $1,500.month, I can do that ion 30 to 45 days. buying and rehabbing. Depending on the area it will take me that long to get a building and use permit to start a construction project.
There is also something to be said for established and mature neighborhoods vs new neighborhood and the inertia of change for both but Ill leave that to the experts...