Technically they don't need to, but storing them vertically means you don't have to surface to launch: if you can make the missiles buoyant (at will, usually by pumping gas or vaporising a bunch of water when you want to launch them) they'll pop out the water like corks at which point the rocket engines fire and off it goes. This means your sub can stay safe and secure below the waves while launching.

Why do you need them vertically stored for that scenario? Couldn't you make the front of the missile more buoyant than the rest, so once you pop it out of the sub it self-rights and likewise pops out of the water?

I imagine flipping from horizontal to vertical would put stresses on the rocket body, so there are engineering problems to solve, but still it's surprising that the only way to solve that problem has to been to build a launch platform 5 or 10 times bigger than is otherwise desirable.

> Why do you need them vertically stored for that scenario? Couldn't you make the front of the missile more buoyant than the rest, so once you pop it out of the sub it self-rights and likewise pops out of the water?

That would make the entire thing more complicated with more chances of failure because now the SLBM wobbles around when it tries to right itself up (on a non-aerodynamic axis so more chance it'll move wrong), you need much bigger openings for the missiles, and your gain will be limited: remember, they're 2m wide so if you stack them by 4 you still need 9m for the missiles alone except now you also need to have 3 rows of missiles meaning you need a lot of extra horizontal space (in the current configuration the missile compartment of an Ohio is about 40m, it'd need to be 60+). It also makes the launch procedure take longer, as the missile can't accelerate as fast out of its casing: it doesn't just pop straight up like a cork anymore.