Is there any in-universe explanation as to why some CAD have to look like guns?
As far as I know, this is mentioned only in the light novels, not the anime:


  The Specialized CADs are often shaped in the form of guns as they have the auxiliary aiming systems incorporated in the area corresponding to the barrel. Co-ordinate data is input at the moment the activation sequence initiates, in order to reduce the calculation load on the user. Psions aren't emitted from the muzzle.


(Taken from the wiki, which takes it from Volume 1 - http://mahouka-koukou-no-rettousei.wikia.com/wiki/Casting_Assistant_Device_(CAD)#Specialized_CAD)