Why does a rocket launch straight up?

The densest part of the atmosphere lies in the first 10–15 km. A rocket that pitched over early would spend far longer in that thick air, paying for it in drag, heating and structural loads on a vehicle that is essentially a thin-walled tank. Climbing vertically for the first ten to twenty seconds gets the rocket through the worst of it quickly and also carries it clear of the launch tower and pad infrastructure before it starts steering.

Orbit is not a matter of height but of speed. At 200 km altitude a spacecraft must travel at about 7.8 km/s parallel to the surface, so that its fall towards Earth is matched by the curvature of the planet. Almost all of a rocket's energy therefore goes into horizontal velocity. Shortly after liftoff the guidance system tilts the vehicle a few degrees, and from then on gravity itself does most of the turning: the flight path curves over naturally in what engineers call a gravity turn, until the upper stage is firing almost horizontally at cut-off. Steering too late wastes propellant fighting gravity for longer than necessary; steering too early wastes it fighting air.

Launch profiles are computed to balance those two losses. Vehicles with lower thrust-to-weight ratios climb longer before pitching over, while lighter, more powerful rockets can start their turn sooner. Trajectories also respect constraints such as keeping expended stages falling into safe ocean zones and, for crewed flights, ensuring an abort at any moment leads to survivable conditions. Air-launched systems and spaceplanes do things differently, but every rocket that has reached orbit from a pad has begun by going straight up, then turned to chase speed.