So my question is, why do people do this.
Because some people are chumps.
That player may actually have a plan and/or be trying to do something, like see how high they can get staying in level 0, or trying to get enough to power upgrade to 1 upgrade shy of level 2, where he'll do it the same thing again. It's kind of like the WOW South Park episode where they level up using the weakest creatures.
That said when many are playing the game (for lack of a better word) "properly", this can end up just looking like a style of Griefer, and lacks "sportsmenship" but try PM'ing the player and asking. Who knows, you might get a responce.