Because Democrats want someone who will run the country and Republicans just want someone who will maintain their minority control. We are concerned for the country, they are scared for their very existence and self reflection itself is dangerous for their existence. Basically self reflection is necessary for what Democrats want to do, and it fatal for what Republicans want. So we do it, and Republicans avoid it at all costs. We get a Kamala who we hold to a high standard, they get a Trump who can punch them in the face and they’ll still support him because of the party.
The problem with that is that, being on the reasonable side, if we just assume the obvious and then are somehow ‘proven’ wrong on a specific topic (there are thousands of them, so it’s bound to happen sometime) we legitimately lose actual credibility in the eyes of people who matter.
Like the ‘they’re eating the dogs!’ things. It’s perfectly reasonable to mock him for it being an issue, but insisting it has never happened and that even the idea is ridiculous, opens the entire side to being wrong if even one crazy or oblivious person of color has ever done it, which it almost certainly has. I mean if you look hard enough, you could probably find a crazy example of that from any cultural group. One example and pretty much all the mocking gets flipped around in the minds of anyone only half paying attention, and certainly from the other side next time we insist something doesn’t happen.