Or, possibly, because it is an augmentation, & thus a secondary weapon system, the costs in time, upgrades, weight & risk to the crew (a missile exploding/burning inside the turret armour is going to be more lethal than if it does so on the outside) of the add-on are not deemed to be worth it & the launch hatches/openings are weak points in the turret.
A tank with an integrated missile system built into it from the start could be another matter entirely. For starters most missile systems could fit into an armoured bustle at the rear of the turret & be raised/elevated to fire (the turret casting/design including armour between the crew compartment & the bustle). This would require a re-arrangement of how most turrets are laid out but could be easily worked into the design, rather than having to fit it ainto an existing lay-out.