... if I had the cash I could buy into a private company via a private placement without seeing a single piece of paper. The extent of DD would be only what I wanted to see.
You would give a private company your money without even seeing and signing a subscription agreement with the terms and conditions of your investment? That would be VERY foolish.
At minimum, there should be at least a comprehensive PPM and a SA when considering an investment in a PP. I don't know how you'd do DD without either of those.