Substitute any other ethnic group and you may see why it is insulting. A look at the Thanksgiving greeting cards in November shows that we tend to single out Indians as acceptable objects of caricature.
You mean the ones that run all the convenience stores and Best Westerns?
Unlike the drunken pugnacious Irishman , or the cheap Scotsman like Janitor Willie on the Simpsons, or the dull Canadian, or the rigid German, the drunken Russian etc etc etc. Grow up.