Because data do not care about sides. If you have different sources from different nations and different entities providing corroborating evidences (satellite images from private companies, survivors interviews from independent journalists, radio interception, videos from front line soldiers) and have thousand of independent lawyers, analysts, activists from multiple nations creating reports of thousand of pages that all agree despite being from non aligned countries and no profit, then there is weight on what is written in those reports.

If the US lies very often and the US says that Russia is committing genocide that does not mean that Russia is not committing genocide. This just means that the US does not lie all the time. If Russia tell the truth sometimes does not mean they do not lie very often.

Explain to me why you feel the need to pic side with a corrupt government when there is no need to do that.

EDIT: just to further clarify: I do not believe Russia is committing genocide because the US said so. I believe it because reliable independent data says so. If the US government agrees with data because it is convenient for them to say so, I do not care. The US agreeing with reality does not change reality. Believing Russia telling that Russia is not committing crimes in their invasion of Ukraine is stupid and naive.