I don’t have a good answer to your question, but to me “white” as an ethnicity makes about as much sense as “Christian” does as a religious description - they both cover such a wide range of backgrounds and beliefs to be essentially useless. Do a Catholic, a baptist and a modern evangelical actually believe the same things beyond how they frame those beliefs?

To my mind, same goes for ethnicity - “white” can mean anything from the baltics to western Europe to north America, and to my mind, is kinda racist. It lumps people from as diverse places as Ireland and Russia together purely based on appearances. I get “black” as a self-selected descriptor of people who do have a big cultural touch-point in common - our ancestors were enslaved, brought here against our will, and we still feel the impacts of that even if our ancestors themselves were from a wide background.

I guess “white” is an easy antonym to “black”, but then that still comes back to a racist tint - “we are white because we aren’t Them” - and lumps in people who have nothing to do with the lasting impact of slavery in the US into this “oppressor vs oppressed” false dichotomy.