Because I taught calculus and physics for years. When I was a student in physics we had to take calc, linear algebra, ode's, pde's, complex variables, vector analysis, and Fourier series and boundary value problems. Now many programs only require through linear algebra and ode's for a physics degree. I'm sure some require more but, in general, much of the math is now learned in the physics classes. Trying to learn, for example, pde's while sitting in an E&M or QM class is not terribly efficient. I noticed, in general, that those with more math tend to do better in their physics classes. Even those who had already completed a couple terms of calculus before taking their first physics class did better than those taking it concurrently.