Physics is the most basic science because it is the science of matter, energy and their interactions. Every other hard science, such as biology and chemistry, is merely a study of a subset of physics' general purview since both biology and chemistry are ultimately the study of certain types of matter, energy and their interactions. Nonetheless, other sciences contain enough specialized information that practically speaking, they are separate.
Because the specifics of various scientific disciplines are so deep and complex, the study of physics focuses on the most fundamental attributes of matter and energy. Where other disciplines focus on the manifestations of physical principles, the effects of the interactions of matter and energy, physics focuses on the interactions and the actors themselves. While a biologist could state that a llama spits in certain situations and the reasons why, a physicist could state the forces involved, the amount of energy contained in the projectile and more.
All other sciences depend, ultimately, on the fundamental substances, energies and interactions physics studies. Even despite this very basic study of the world, the field of physics is still vastly complex. Many different disciplines of physics operate somewhat independently, with expertise in each requiring years of study.