It's not about gross incompetence; it's about a jury's perception of incompetence, colored by their perception of the doctor and their perception of the grieving family members. If it comes to a jury trial, the court relies on the jury to make legally factual findings, so the courts end up relying upon the statistical abilities of the median juror, colored by the aforementioned biases.
There are medical boards to remove incompetent doctors, but those influence jury trials primarily through submissions of findings as evidence for consideration by juries. A good lawyer will portray medical boards as a bunch of doctors biased against passing judgement on fellow doctors.