I think you're describing single-family, but bulk of those deals are done in multi-family space, where a building is either an apartment complex (with units intended for rent) or a condo (with units that are owned). So most real estate investors are buying from another investor (or a builder).
While there are some developers that do condo conversions (apartments -> condo), apartment conversions are rarely a thing, probably due to complexity of the deal.
Also, from the article it sounds like a lot of these would-be landlords/investors are having a tough go of things, and aren't in fact able to turn a profit which is why they have to foreclose. Your characterization of landlords as parasitic leaches taking excessive profit without doing any work or risking anything isn't super consistent with the fact that many landlords don't beat the market, or even take significant losses.
[0] https://arstechnica.com/tech-policy/2022/10/company-that-mak...