Why do Immigrants in the USA Speak with Foreign Accents?

Many people sometimes are asking me: "Why do you speak with accent?" Indeed, why do I ? My children, who come with me, do not speak with accent. No one ever thought that they are not Americans. Why even British, who lived in the USA for a long time, still speak with their British accent? Why they could not change their British accent for the American one? I have the answer that I share with you in this story.