How come Arnold Schwarzenegger still has his German accent after living in the US for over 40 years?

Why is that Arnold schwarzenegger still maintains his thick Austrian-German accent inspite of living in the USA for over four decades and his having an American wife? Why is that Heidi Klum has lost her German accent and speaks near perfect American English?

Is it a generational thing?

First, Heidi Klum’s accent is very noticeable.

Also, Klum was 19 or 20 when she moved to the United States (she was born in 1973 and moved here in 1993) and probably knew quite a lot of English at the time. Schwarzenegger was a little older (21) and spoke very little English when he moved here.

Klum’s accent has reduced substantially over the last couple years. It is likely she has taken diction lessons to help her in pursuing her acting career. This would be more likely necessary for the roles she has pursued than those schwarzenegger has pursued.