that just depends on the person and how much effort they actually put into it, i guess the people you know just don't care which is just sad. not knowing the language of the country they live in properly only gives them trouble but they are responsible for themselves.
in global comparison german is actually one of the easier languages to learn.
i know many people who only lived here short-term but speak german extremely well. lately i was impressed when i met an arab/afghan dude who was buying school books for his daughter, i spoke to him and he said he has been in germany just for about 1 year but he spoke like perfect german.