Most of the tech for the Minuteman I was developed in the mid-1950s.

With that level of processing, would you rather solve a 2d problem by precisely orienting the missile before launch? Or a 3d one by requiring it to orient during flight?

Keep in mind: any equipment to self-orient in-flight also needs to be carried on the missile itself, while being tolerant of launch, acceleration, and reentry forces.

Any precision machinery at the launch site has no such requirements.