Because this bug.
Once more, no, it isn't. You have to do something remotely intelligent to earn RANKING points, not blindly press a single key and wait for the rewards to drop.
Ranking usually is a way to measure the difference in skill level between several players, by judging their performances and awarding them points based on this. Pressing one key, as you might have seen by looking at the bottom right-hand corner of your screen, doesn't account for much in this since it is incredibly easy.*
So, one more time (sing with me !) : try and use any kind of skill (Inferno, Rolling stone, Earthquake, Fire rain, Spike wave, whatever comes to mind) on a crowd of monsters while closely looking at your ranking points. You might witness a noticeable difference after you do.
*Yes, pressing A to summon a Fire emblem is easy, too, and awards ranking points. Call that "god's power".