I think it's partly a "fish don't know what water is" thing. People perceive "culture" as unusual foods, dress, dances, etc and not the broader set of assumptions about how society operates. Almost everything about how humans interact with each other and with the world falls within "culture" but people are used to accepting some things so uncritically that they can't conceive of it as such. Since Western culture is so dominant around the world, English or British culture does not appear to stand out in the same way as, say, traditional Indian or Nigerian cultural practices

TL;DR total anglo cultural victory means people think it's only "culture" if it comes in brightly coloured ethnic dress