Why is solvation considered a surface phenomenon?

Solvation is a surface phenomenon in the sense that it starts at the surface of a dissolving solid.
During solvation, the particles of a become surrounded by particles as they leave the surface of a solid. The solvated particles move into the solution.
For example, water molecules pull sodium and chloride ions from the surface of a sodium chloride crystal. The solvated Na⁺ and Cl⁻ ions end up in the solution.
We also use the term solvation when water molecules surround polar groups on the surfaces of cell membranes.