Is +6 really the most stable form of sulfur?
I think it's a matter of activation energy and reaction kinetics. Although the product is much more thermodynamically stable than the reagent(s), the reaction path could feature a high activation energy which requires those harsh conditions and even a catalyst to make it happen. As shown in the graph below, despite having a favourable difference of free energy ($\Delta G &lt; 0$), the activation energy $E_a$ required to go from reactants to products could be not negligible and hard to overcome.



The catalysts allows the reaction to happen more easily by lowering the free energy of the transition state (by stabilising it). The activation energy is also reduced by the high temperature, according to the Arrhenius equation:

$$k = A e^{{-E_a}/{RT}}$$