E.g., find and the the van der Waals EOS for the fluid. In many cases you need to go back to the beginning, i.e., guess a new functional form. This is because you can't fit propane and water with the same functional form. Have to start all over again for many new fluids. Extrapolation outside the region of parameter regression is not valid.
Extrapolation often gives unphysical results.
You don't know anything about many of the other properties. If matter is truly composed of molecules, then it must be the interactions among these molecules that give rise to the macroscopic observable behavior. If we can understand the microscopic interactions we can predict the macroscopic behavior. This is the goal of statistical mechanics.