Neophyte question: Why do investors have to ask founders who else is investing? Is it not possible for them to check this themselves, via public records? [There are some sites that even pre-package the SEC filings for consumption by journalists, etc.]

It makes perfect sense to me why investors would want to know who else is investing, for a number of reasons. For one, standing on the sidelines knowing that most startups will fail is not a reasonable strategy. This is because some startups, no matter how unlikely, will succeed, and some VC will, despite our better judgment, have invested in them. If other VC deliver higher value for their clients than we do for ours because we cautiously and prudently stand on the sidelines, then we stand to lose the confidence of our clients.

But I do not understand why anyone would rely on the statements of founders to determine who else may or may not be on board as investors.

The way VC and angel rounds work, is in rounds...

Each round has investors commit and before they close, some investors may ask who else is investing. Most closed rounds on really early stage companies are hard/impossible to find, but all pre-closed rounds are impossible to find since there is no record of something that hasn't happened yet.

Here's one website I was thinking of: http://formds.com

Is the information disclosed in Form D's severely limited or useless for determining who is being funded and who is funding them? If yes, then what is the purpose of this website?