It doesn't feel intellectually honest to suggest that expert witnesses are there for any other reason than to promote the case of the side that hired them. They rarely outright lie, but "try to make facts fit a predetermined conclusion" is their job description. They would not be brought to the stand otherwise.

I agree that there are ethical problems around helping people twist facts for their own purposes. My point is that everyone in a courtroom except the judge and jury are there to do exactly that. They make the strongest possible case to get their desired outcome from the available facts.