Because Sacha Baron Cohen takes this prank expertly too far. It's seems so accidental, and is so terrible, that you don't think he trying to fool people. I gasped at first.
Cohen was accepting the Charlie Chaplin Britannia Award for Excellence in Comedy. Presenting it was Selma Hayek and a child actor who worked with Chaplin, with a present for Cohen. We'll let the video take it from there.
[H/T Huffington Post]