That's just to simplify the math for learning and for modeling one component of rocket flight IN ISOLATION from all the many other complicating factors. Once you understand and can accurately computer-model rigid projectile flight in isolation in basic scenarios, you can then start incorporating other factors into your model. Other components likewise have to be studied and modeled in isolation to achieve accurate models for that component. Eventually you achieve a comprehensive rocket trajectory model by combining all the thoroughly-tested-in-isolation constituent component models. This is a standard approach in numerous fields of science and technology. For long range and intercontinental rockets, curvature of the earth is taken into account when calculating trajectory, along with coriolis effect, magnetic field anomalies, wind, air pressure and various other factors, dynamics of liquid payloads, like fuel, etc.