Because "in stitches" is a very amusing phrase
in English, as once used by Vladimir Nabokov
in the following quote:

"Upon being challenged to read Eugene Onegin aloud,
he started to do this with great gusto, garbling every
second word and turning Pushkin's iambic line into
a kind of spastic anapaest with a lot of jaw-twisting
haws and rather endearing little barks that utterly
jumbled the rhythm and soon had us both in stitches."