Consider if the F-4/Sparrow were on rails. The front missile can't launch
without burning the aft missile, and the aft missile can't launch because
the front missile is in the way.
(Not counting the fact that two of the fins are recessed into the
fuselage,
and the missile body itself is in a slight recess)

Originally the plan was to use rails that swing down... the front missiles
were on a trapeze system, and the aft ones were on arms that swung down from
an inboard hinge (parallel to roll axis). That got scrapped halfway through
the design process.