@saint For the same reason that it would be offensive if someone used the same accent to demonstrate a conversation with, say, a gas station cashier. It is a very biased, American view to single out a race as call center workers when in reality, very little of the Indian population actually does this, and there are many other nationalities involved in call centers, including GASP Americans. While it does have an element of truth to it from an American standpoint, it is a racial stereotype that often has negative connotations to it in our society, and referencing it for anything therefore has the potential to be offensive.
It was offensive in this case because it was completely unnecessary to single out any particular race; it served no purpose and added nothing to the VP’s points, other than to sneak in a mildly racist/stereotypical joke or comic relief, as evidenced by Biden’s abrupt change in manner.
However, all things considered, the comment is ultimately only mildly offensive; he did catch himself quickly and refrained from saying anything blatantly racist. leaving only a bad “aftertaste,” if you will.