The secret is that Lemmy is tracking karma, it just doesn't show it. There are fields in the database that hold the total comment score and total post score.
Yeah I recognize it's a decision from the admins here, but it's one I approve of heavily for reasons said when I posted this to the fediverse channel. The motivation behind posting is less tainted this way.
Nah I don't think there is an option to show it in Lemmy. It just seems it's there in the database either in anticipation of adding the feature, or because because of changed plans.
All the calculations are being done already, so it wouldn't be saving and server time. I think it's just something they might have done because if you decide to do it later I bet it gets a lot more difficult to retroactively calculate it.