The issue is that catalysts are typically formed into fine, spherical pellets to maximize the surface area of catalytic material exposed. This is because catalytic reactions are characterized by an intermediate reaction between the reactants and active sites on the catalyst. As a result of their being made into pellets, a variety of things can occur that reduce the active surface area. As a result of temperature and pressure, the pellets may adhere to form larger particles, which will hence have lower surface area. Additionally, chemical entities present in the reactor may physically adhere to the pellets creating a diffusive barrier to the catalyst. The catalyst can probably be recycled after its effective lifetime, but the cost is certainly not zero (probably similar to production costs in ore refining).
Additionally, although catalysts are not consumed <b>in the reaction they catalyze</b>, they may take part in reactions other than the one of interest. In this way, catalytic material may degrade over time, although platinum is fairly inert.