Assimilation to a certain degree is necessary if you want to be part of society. If you do not speak the language, don't interact with the locals, etc. you're missing out on most of what your new country has to offer and will remain a stranger, often isolated and excluded by your own doing.

This is especially troubleing for children of parents who decide to completely live in their local bubble of immigrants. Speaking the local language well, attending a Kindergarten to learn essential skills like the other kids and socializing with them, as well as generally feeling like they are part of society is pretty essential for further success. If the child does not know the local language by the time it enters school, the parents don't understand or concern themselves with the school system and cannot help/ teach their child at home, the child is not allowed or able to socialize with the locals, be part of local clubs and the local community in general, they will encounter a massive disadvantage to almost all areas of life. They will have a hard time making it through school, a much harder time finding a job because of that, and thus are basically stuck in poverty with little to no connection to the rest of society.

You don't have to give up your entire cutlure, language, food, etc. but simply take on the additional basic traits of the new culture like language skills and customs.