Also, what happened to the attitude that published studies are often wrong?

Even if published studies are often wrong they still constitute evidence in favor of the conclusion they present - just weaker.

Where did the skepticism go?

Got beat up by the arrogance and the phenomenon whereby some kinds of subject seem to be limited to the realm of experts or professional while others are considered fair game for everyone to have an opinion on.

Why do LWers suddenly think they understand how parenting works???

Most of us have seen it happen. We've probably at least got the basics down.