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The shape of the V2 was modelled after the shape of a bullet or artillary shell, which were the only supersonic vessels that the people in the 1930's had any experience with. Rockets don't need boattails because their exhaust fills in their wake at subsonic speeds and concepts like wake and rounded shapes don't apply at supersonic and hypersonic speeds. At supersonic speeds a sonic shock wave is formed at the tip of the vehicle and at each point where there is a change in the shape of the vehicle. For this reason the most efficient shape for a supersonic vehicle is a sharp pointed wedge, which will produce one shock wave at the tip and second shock wave at the back of the vehicle.
Rockets spend the bulk of their time in the supersonic and hypersonic realms so subsonic aerodynamics plays little part in their design.
Rockets initially fly straight upwards to get above the bulk of the atmosphere and its drag as quickly as possible.
The X-15 was suborbital, reaching a maximum altitude of 600,000 feet in its test flights. It didn't carry enough fuel to enter earth orbit and wasn't designed to do so. It was designed to test vehicle aerodynamics and flight controls in the supersonic and hypersonic realms and to test flight controls transitioning from atmosphere to vacuum and back.
Where else are you going to start but at ground level? You can either go all of the way to orbit via rocket power or use a carrier vehicle (air-breathing aircraft or helium balloon) to take you above part of the atmosphere before continuing the rest of the way on rocket power. But this latter approach adds additional complexity in the form of a second vehicle and the limited weight that can be carried aloft by this vehicle. There's no way that a 747 could carry the Space Shuttle and its load of fuel to 30,000 feet, but it can carry and release the much smaller Pegasus rocket that can carry small payloads into low earth orbit.
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