Why is the gravitational force on a point mass outside a hollow spherical shell equivalent to that of a point mass at it
For a uniform spherical shell, the gravitational forces from all parts of the shell combine such that the net force on an external point acts as if the shell’s entire mass is concentrated at its center, due to symmetry.
Ref: Halliday & Resnick, Fundamentals of Physics, 11th ed., Chapter 13: Gravitation.