They easily could (I think it would take the average HN user no more than a couple of hours to implement this functionality), but they won't, because they're very reluctant to make changes.
This is also why HN still looks like a site from the 90s, instead of New Reddit (thank God).
It’s not all bad. We still have HN as it is now. Status quo is its own kind of success. HN is essentially feature complete, so I don’t blame them for not trying to fix what ain’t broke.
> How does ranking work?
Great question, it's something I'd love to know more about as well.