They realize that attempots by government to do so do not really represent an effort to "help", but much more, and effort to "control".
In addition, pre-existing conditions themselves vary critically in degree, type, and impact. Trying to paint all with one brush is over simplistice and does not allow for the ingenuity of the free market.
Certainly there are pre-existing conditions that can be treated, and done so effectively and efficently and within profit margins when realizing that to allow it to go further will only cost the individual or the insurance company much more. Preople also realize this, and therefore it would be possible to create "plans" depending on the particular situation, where a company could charge for covering treatment in such a way where both can benefit.
All sorts of cancer fall into this category depending on when they are "caught", but which also would qualify as "pre-existing".
As I have stated...the best way, IMHO, is clearly to let the free market address it and keep the government out of it as much as possible.
But now the arguement is getting circular. I have made my points and am content to let others read them and make their own decisions.