Theorists came up with the mathematical tools to describe microscopic particles (atoms for example), namely the Schrödinger equation. It’s been verified countless times that this equation agrees with experimental results (Stern-Gerlach experiment for example) and as materials consist of these microscopic particles, it’s only logical that the same equation applies. So, in materials science one can apply these already verified equations, in more complex cases (solids), which is simulation, as this boils down to just solving the equation(s), not developing them.
Because, as an outsider it looks like developing new materials is kind of creating new physics. Also, laymen may just not know exactly what theoretical and computational physics entail exactly, and describe every type of physics that is not done in a lab as theory.