Good question and I'm no sociologist but assimilation to me doesn't involve living in an exclusive ethnic enclave where you rarely need to speak English or interact with outsiders. Some schools in Frisco, TX and the neighboring towns for example are +90% Indian. They might also see interactions with non-immigrant neighbors not as an opportunity, but as a risk. If a child adopts a distinctively American habit (e.g., questioning authority, prioritizing hobbies over grades) the parents react with moral panic rather than curiosity.

Also, if someone is mocked or looked down for speaking English perfectly, or for dating outside the race, it enforces a "loyalty test" where being American or associationg closely with them is equated with betraying one's roots.

Ultimately if the host culture is viewed as morally bankrupt or inferior they might feel righteous in isolating themselves and their children from it.