Because America is a white country.
The fact that our demographics have been changing as a result of mass (mostly Hispanic) immigration since the 80's doesn't change that fact or magically "muttify" all European Americans who mostly come from British, German & Swedish stock.
Britain is 44% Caucasian.
I assume we can agree that no British people live in London anymore, only alien looking creatures who falsely believe they are still British/European ethnically?
This meme is funny at times but it holds no logic behind it.
Changing demographics do not automatically change significantly the makeup of the native peoples when statistically only a small percentage actually end up dating outside their ethnic group and even as changing culture increases that percentage over time its still a small minority and largely only in the minority of areas that have become highly diverse (certain parts of big cities).