That's not how it works. You can't just buy a tract of land. You have to buy the land, jump through hoops to get permits to build, defeat NIMBY lawsuits, get the local municipality to run services, defeat more NIMBY lawsuits, get new permits from a new municipality administration, then finally break ground on construction.
NIMBYs of all stripes throw up roadblocks during the permitting process and will then sue to get an injunction. If you defeat those lawsuits they'll go after the municipalities suing that permits were approved illegally (which they sometimes are).
After several rounds of legal wrangling you can finally start construction. This ends up with only huge developers being able to build because they can absorb all the pre-construction costs until they sell all the homes. They can also afford the scale to benefit from bulk orders and contracts.
The cost of expanding housing is mostly in the project development rather than labor costs of construction. Even if you had construction robots the construction companies would only offer marginal savings as their bids would just be human contractor - 1%. There would be no reason for them to leave money on the table offering a huge discount off human labor.