Because otherwise, the very-much alive movie star might unleash hell on you, the way he did on your sidekick Guillermo, who tonight dared to ape Damon's character from The Informant!, right down to the fake 'stache and cheap suit.
With Informant! costar Joel McHale standing idly by, Damon and Guillermo went to town on each other, movie-star-style, after Damon caught the portly parking-lot attendant posing as him and dared to question his thespian credentials.
"Is that my mustache?" Damon demanded, ripping it right off Guillermo's face, after which Kimmel's loyal stooge sent the best lover Sarah Silverman ever had flying right into a table.
When all was said and done and the apple chunks had been knocked out of McHale's smug mouth, Damon let it be known who he really blamed for all his troubles in life.
"I'm going to f--king kill Kimmel," he vowed.
Remember why Jimmy Kimmel is uncomfortable around Matt Damon in the first place? Refresh your memory right here.