supercomputers are specialized products with fast networking to enable real-time updates between nodes. The total node count is limited by the cost of the interconnect to get near-peak performance. You typically run one very large simulation for a long period of simulation time. folding@home doesn't have the luxury of fast networks, jut lots of CPU/GPU. They run many smaller simulations for shorter times, then collect the results and do stats on them.

I looked at the various approaches and sided with folding@home. At one point I had 1 million fast CPU cores running gromacs.