Why are so many people obsessed with immigrants culturally assimilating?

I'm English and in my lifetime people have really leaned into the same arguments as France, that if people want to live in England they need to be British. Which I guess would be defined as taking on British values ditching any aspects of their own cultures, eating English food and dissociating themselves from their former state and history. I guess I'm curious about this mentality, from my perspective people tend to assimilate in a few generations anyway so screaming at 1st generation immigrants that they need to conform to fit in just comes off as bullying but I would be interested to hear what people think on the subject, all nationalities welcome.

Why are so many people obsessed with immigrants culturally assimilating?
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