That's an interesting observation and I think it comes down to immigration policy. I haven't actually looked into it but I've heard that Japan basically doesn't allow for long-term immigration, except probably in exceptional cases like PhDs.
Where EU countries (I know this excludes the UK but it didn't for a long time) allow easy long-term immigration by EU policy. Even with Brexit, I don't think that culture of easy immigration is going to just up and disappear. So having a culture and/or policy of easy immigration alongside "well, actually, not those guys" where "those guys" includes anybody who's not already culturally/ethnically part of the nation is, minimally, counter-productive and perhaps a bit hypocritical.