With so many people already giving the reason behind why Polite Indians do that, I need not repeat them.
But I will like to add one very important point.
India is a huge country - Population wise. They have millions of Software Engineers I guess.
You cannot expect all of them to be Experts or have lot of knowledge. But they do have a substantial amount of talent. I am not sure, but the percentage ratio might be as good as that of USA or UK or any other country.
Its probably their huge numbers, that you feel that "In almost all cases it appears that these folks are from India."