Because if we set out at the onset to dictate what constitutes “too much” density, well, many of the commenters in here are of the opinion that even rowhouses are too dense.
You’ve nailed the crux of the problem right there. And yeah, like with everything else with human beings, you’ll get a big range of people who have different tolerances for density.
But besides their own individual opinions of what is too much density, there’s a biological/psychological definition as well, that all humans in common have, and that’s what the scientists were studying.