Also may "experts" are specialised to a field with maybe a general knowledge of others so just because you get a lawyer/doctor etc... answering, it doesn't mean they are all knowing up to date across the whole scope of law/medicine etc... which may well be misleading to people who think just because they are a lawyer/doctor/nurse etc... that their opinion is more important that others (which may be more correct) even in areas that aren't their specialty (if that makes sense?).
Helping with general advice under a disclaimer is one thing, putting your professional reputation on the line and the far reaching liability issues which would also come with it is another, especially with all the hindrances of only knowing what is told by an anonymous stranger on a Q&A site - most proper professional advice would usually be given with far more information, questions, enquiries/tests etc... along with appropriate safeguards and indemnity.
Trouble is also that people would then tend to target specific posters to ask questions and pressure could be put on them to answer questions, it be almost expected and them even getting stalked round the site, rather than it being a general Q&A site with input from some experienced people at their own leisure (which is fantastic and something I've benefitted from).
It would be a liability nightmare!