Why can’t subs just attach torpedoes to their hulls like missiles under the wing of an aircraft?
Pros; The torpedoes are on the outside so there’s no loading and filling of tubes which reduces firing times and keeps the noise down.
Cons; Risk of damage to torpedoes and maybe one going off from a bump.
You will of course surmise from all this that I know little of submarination, but the sub threads on here always fascinate me 
Con: Can’t service torpedo if needed
Con: Causes all sorts of cavitation in the water, which makes the sub stand out like a Christmas tree. This is bad.
Con: Why the need for rapid fire torpedoes? With today’s homing shots, one is all that is needed per target. It’s not like you are strafing a fleet.
Con: What do you gain by putting them outside? A little more legroom for the Captain? There isn’t really any reason to, and all sorts of negatives involved.
The first thing I would think of would be drag. If you fired one, then you’d have asymetric drag.
The second is weapon load. The Los Angles class attack boat carries 26 tube launched weapons. And those suckers are pretty big. I don’t think there is physically room to arrange that many all over the hull.
Another con: I don’t know if they are still designed this way, but torpedoes used to have a little paddlewheel device that measured how far the torpedo had travelled. It wouldn’t arm itself until it had covered that distance. If the torps were outside…
Another con: Any projections on the hull of the sub add turbulence. This turbulence, in turn, adds to the noise signature of the vessel, making it easier to track via passive sonar. The smoother the hull is, the quieter the sub and the less detectable it is.
If you’re chasing down a renegade captain whose sub is coming head on to yours, it could be an advantage 
I think I get it though, the main reason is sort of like a sports car with pop up head lights. You could have them popped up all the time during the day but you’d just end up getting lousy MPG and look silly 
Submarines have been known to carry these things around on their aft deck though.
Only because there’s no place for them inside. New subs (Virginia class) were designed specifically with seal deployment in mind.