Each of the representative bodies is invited to put people onto the various committees. There are more committees than people usually and so a call goes out to the ranks for suitably qualified people that understand the issues and are accustomed to working collaboratively with a wide variety of views.
The vast majority of this work is undertaken by volunteers usually at significant personal expense to work towards the goal of a better GA.
Don't use these forums as metric of the calibre of people working in the real world as all to often the written world is poorly interpreted.
Not to mention:
As these are anonymous forums the origins of the contributions may be opposite to what may be apparent. In fact the press may use it, or the unscrupulous, to elicit certain reactions.