1. People yearn for identity labels. It's a core part of human existance.

2. "White" and "American" are problematic identity labels. People therefore often reach back toward European ancestry (real or supposed), for identity labels that are less controversial.

3. The average person isn't aware (or concerned) that "Viking" isn't strictly an ethnicity. Because it's nevertheless a commonly used identity label.

Not everything has to be an opportunity to spot Nazis hiding behind every tree, or showcase your pedant chops. People wear shamrock jewelry or put an Italian flag bumper sticker on their car because it's fun and feels good, simple as that. Only a small number of legit white supremacists, and a legion of absolutely insufferable Internet progressives, think about this all that deeply.