The fact that it can be changed arbitrarily. You can assign whiteness the same way you can assign blackness 8n this country.
Why do you see so many hispanic/Latin people come to this country and deny blackness when it's clear they have African heritage ?
Why did Irish and Italian immigrants have to earn their whiteness?
It's not real breh
I agree in the sense that race is socially constructed and changes based on era, circumstances, etc. -- it isn't a biological fact and is a fairly recent social invention (within the last 500 or so years).
But it is a very real social phenomenon, so people get incredulous when folks use the language "race isn't real."