Physicists continue to use Newtonian mechanics because it provides accurate and practical descriptions of the motion of macroscopic objects under everyday circumstances. While Newtonian mechanics is indeed a subset of both quantum mechanics and relativity, it is often sufficient for many applications where the velocities involved are much smaller than the speed of light, and the objects are not extremely massive or extremely small.
Newtonian mechanics is simpler and more intuitive compared to the other two theories. It provides a straightforward framework for understanding and predicting the behavior of objects in the classical limit. In many cases, the effects of quantum mechanics and relativity are negligible and do not significantly impact the results obtained using Newtonian mechanics.
Furthermore, Newtonian mechanics often serves as an approximation or an excellent starting point for more complex systems that require a more sophisticated treatment. For example, in celestial mechanics, where the motion of planets and satellites are considered, Newtonian mechanics is typically accurate enough for most practical purposes.
Quantum mechanics and relativity, on the other hand, are more comprehensive and encompassing theories that describe the behavior of matter and energy at a fundamental level. They are applicable in domains where Newtonian mechanics fails to provide accurate predictions, such as at the atomic and subatomic scales or in situations involving high velocities or strong gravitational fields.
In summary, physicists continue to use Newtonian mechanics because it is a reliable and effective tool for understanding the behavior of macroscopic objects in many everyday situations. However, when dealing with systems involving extreme conditions or tiny scales, the more comprehensive theories of quantum mechanics and relativity become necessary for accurate descriptions and predictions.