Because they are, as well as materially motivated.
So if you're just repeating the claim, there is no point. Say something new?
What denotes serious vs non-serious government worthy of weight?
So the statements of the Israeli government would not have much weight in this, as they have obvious incentive to lie. The government of Russia should not have much weight, because it wants to whitewash its war crimes in Ukraine. The government of the US should not have much weight, because it has been eviscerated of everyone of any intellectual capacity.
as if they are neutral observers
They are not neutral observers, but (some of them) make serious statements and are capable of responding to facts even when it concerns an ally. We don't see that with the US. We do see it with the UK, so even though it is not neutral, it forms part of the lack of consensus.
Going on the basis of consensus means that sometimes Wikipedia will not state as fact something that is a fact. And that's fine. It's better than the alternative.