It's in part because we think race is important.
We don't notice people's teeth, unless they are unusual. That's why I can watch a film I say 40 years ago, and say, oh my gosh, their teeth! But I didn't notice them at all when I saw it the first time - everyone's teeth were like that.
If, 1000 years in the future, we are all effectively mixed race and no one thinks much about it, and the concept of race as it exists now is no longer a thing, I imagine no one will care much in historical productions either.
That's not true now. In fact the effect of modern identity politics is that we think about and notice race more than ever. We are even told that in some cases, we must absolutely not have cross race casting. So of course we notice it, whichever direction it happens in.
It's like Morgan Freeman said, if you want people to get over racism, stop talking about race all the bloody time.