Fuck off back under your rock.

We like flags, man. Flags are cool. Besides, it isn't just a western things, flags are a human thing. Almost every culture out there uses flags and/or banners.

For that level of specificity, you're going to need not just flags but https://en.wikipedia.org/wiki/Handkerchief_code

Flags are both literally and figuratively a banner for people to match under. When people protest flying a flag above them is an effect way to ensure anyone who sees them understands their motivations.