can't speak for everyone, but personally:
too many misdiagnoses (later figured out with friends/reading/randomly overheard tidbits in the paper/etc.), lifelong
too many doctors, same from both genders, who wouldn't listen to me or were threatened by my intelligence
clear history of medical "science" thinking women were messed up men and children were to be managed for adult convenience
too many obviously unscientific medical teachings preached to me
but probably most importantly, doctors are trained to see everything as a problem to be solved. much of what I love to read here is that with a quick question, moms can find out that--even if their kid has a problem that does need a medical solution--other moms and kids and families have had the problem, too, and here's some ways it was dealt with ... and that makes it so much less scary to face (the situation at home--and a doctor, if one is necessary)
what a fast way to get an incredibly diverse set of opinions from people who really care and often have done significant study (in life or class or reading)!
And to be frank ... I used to work in medical statistical research. Doctors don't know SO much more than a layperson with an interest in medical reading, and certainly not too much more than a nurse pracitioner, unless one gets into arcana. If you have an area you have studied and you understand biology pretty well, you are likely to have a better grasp than a non-specialist about your area of interest. Plus, you won't have $100,000+ of education you are trying to subconsiously justify to yourself by clinging to what you were taught there.
Doctors are very useful. But one needs to be informed of the full range of ideas, or we would all end up with surgery for a trigger finger (massage, two hours, solved). And lots of the advice from lots of moms is, Go see a doctor. So I'm sure it all works out.