Why 你在笑什么？, why not 你笑在什么？
「在」in「你在笑什麽」is not equivalent to English at in what are you laughing at?. To demonstrate by analogy:


你在吃什麽 - what are you eating?
你在做什麽 - what are you doing?


「在」is actually equivalent to the suffix -ing in laughing, eating, doing. It is English which grammatically requires something as a target for the verb laughing; this requirement is redundant in Chinese.


  你在笑什麽
  
  Literally: You (你) [action in progress / -ing] (在) laugh (笑) what (什麽)?


Perhaps it is easier to see what's being omitted in English.


你在笑什麽 - about/at what are you laughing?
你在吃什麽 - what [food] are you eating?
你在做什麽 - what [action] are you doing?