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.
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.