Why are all space rockets launched from the surface of the Earth launched vertically, although at the dawn of cosmonautics

Why are all space rockets launched from the surface of the Earth launched vertically, although at the dawn of cosmonautics it was assumed that such launches would be made from inclined platforms?

With an inclined launch, the rocket moves longer in dense layers of the atmosphere and therefore the energy loss to overcome the air resistance forces is greater. In addition, the vertical takeoff launch complex is more compact.

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