Gross negligence

When you take out a policy with any insurance company, you are covered for negligence and even 'gross negligence'. Why is it then, that when you are in an accident and even if it's your faulty as you were negligent, surely you should be covered, but the insurance company decided which claim they will pay out and which not, as they are according to them, the experts. Even if they weren't involved in the accident, they think they know what happened and they are qualified to make a decision whether the claim must be paid or not. Who makes them the experts when they were not there. Negligence or gross negligence is covered in any/all policies and therefore a claim can't be denied on this basis