The "Good reason", is that an equation, say,
L = 1/2 * rho * Vel.^2 * area * (alpha*P1 + P0);
Is far quicker to read and write than:
Output.Lift = 1/2 * Environment.Density * Aircraft.Velocity.X.^2 * Aircraft.WingSurfaceArea * (Aircraft.Alpha * Aircraft.LiftCoefficientP(1) + Aircraft.LiftCoefficientP0)
Structures (particularly large ones, and arrays of structures) can be much slower than basic variables.