Well, giving Karma is takes some time. It is much easier just to write and read posts. Giving +Karma (or -Karma) is an additional task.
Imagine the live of a merit source in BTT lol.
Anyway, I try to give karma as much as I can (my time is limited as well, but I try to give much karma, just like merits).
Open Karma logs would also solve this, imo. People would give less karma for people who doesnt give karma to anyone...