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As already mentioned, it only took six hours. To make it run on a cluster or supercomnputer, you must parallelize the algorithm, which will take considerably longer time, even if it is easy. Then they likely have to apply for access, which also take time. Then it is easier to just run it on an available computer.

Because it only took 6 hours anyway.

For the same reason you don't see word processors using a more powerful computer or cluster to render text for display as pixels.

- It would be awfully inconvenient to program

- It would require buying, building or obtaining access to such a machine

- It'd require investing some amount of time and/or money -- obviously completely unnecessarily -- because whatever desktop or laptop happened to be within reach is perfectly adequate to the task