Given their goal really is just to delay/stop entropy, then the energy simply being created from the process and then released into the universe is all that is necessary for that goal, so there isn't a need for Kyubey to collect it. While Kyubey assumed keeps "collecting/eating" the used up grief seeds, because witches being revived doesn't create energy the same way as the transformation does, thus it wouldn't be in his interest to allow it.
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