Hi Kevin: That is a very good question and one that's asked frequently.
It's not that it's incapable, it's simply because FEA codes are not typically designed to solve the rigid body motion case. They are designed to solve elastic body response. You can solve rigid body motion response with a solver that's designed for it, such as SolidWorks Motion, which comes with SolidWorks Premium - it is a rigid body solver.
Precisely what happens in an FEA solver is: because of lack of sufficient restraints, there's one or more zero's on the main diagonal of the stiffness matrix, and this means the matrix is non-invertible. Ref. from matrix algebra, or linear algebra texts - zeros on the main diagonal is analogous to dividing by zero, which has no solution. The zeros get in the matrix because there's not enough body restraints to mitigate RBM (or rigid body motion).
You can however "fool" the solver into inverting the stiffness matrix (and solving it), if you turn on the "soft spring" option in the Properties of the study, and use the Direct Sparse Solver. This switch actually places small values of stiffness into the matrix where previously there were zeros - making the matrix invertible so it can be solved. The Soft Spring option physical equivalent is like packing your model into a soft foam enclosure. For example, photographic equipment you may see packed into a squishy foam inside a hard case for carrying or shipping - the foam prevents the camera components from shifting as rigid bodies, but the foam is so soft relative to the camera parts that it won't cause elastic deformation of the much stiffer camera components.
This Soft Spring option is useful, for example, to discover the elastic body vibration mode shapes of bodies suspended in free space or air such as satellites, missles and airplanes.
For your case with the box sliding on the plane- the SolidWorks Motion product would be best used to solve the response of the box to a side force, as a rigid body. Good luck.