What is up with sinosphere “plagiarism” claims

Recently, I have seen many Chinese, Korean, and Vietnamese netizens accusing each other of “plagiarism" in terms of clothing, food, even holidays and how their society works. It is almost like the last 1000 years of history has no relevancy to today. Is there a belief that these countries just spawned out of nowhere, or history is taught censored? Or the concept of cultural exchange no longer exists? I ask this as someone of chinese-vietnamese descent, and recently has been receiving discrimination from both sides.