This is a common philosophical/ideological difference.
The left tend to argue that impartial people exist, and that they are numerous/easy to find.
The right tend to argue that there's no such thing as an impartial person, that you can get people who start out partial and do their best to be fair (e.g. judges) but it takes constantly training, reinforcements and incentives to do that and there's always the danger of slipping back. You definitely can't assume it.
The adversarial court system is based on the right-leaning belief: although expert witnesses are told to be impartial, the system doesn't assume this is enough and so witnesses are called by one side and cross-examined by the other. The lawyer's job is to sniff out any signs of bias or incompetence.
The left tend to argue that impartial people exist, and that they are numerous/easy to find.
The right tend to argue that there's no such thing as an impartial person, that you can get people who start out partial and do their best to be fair (e.g. judges) but it takes constantly training, reinforcements and incentives to do that and there's always the danger of slipping back. You definitely can't assume it.
The adversarial court system is based on the right-leaning belief: although expert witnesses are told to be impartial, the system doesn't assume this is enough and so witnesses are called by one side and cross-examined by the other. The lawyer's job is to sniff out any signs of bias or incompetence.