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Because:
1 - Comments increase activity. It keeps it high on the sort by "active" result page.
2 - More comments make an item look more valuable from the preview page.
3 - Many users over time will end up being followed around and their activity will be copied by bots or copycat humans.
4 - It gives the author a reason/opportunity to engage and add to the comment number without looking spammy
5 - It makes your article look more valuable when people click into it to read it.