Jergmo, D&D isn't a simulation. There are loads of things in it that don't make sense. It doesn't make sense that killing monsters helps you to learn new spells faster in the first place, so saying that the way that monster races modify this is nonsensical is a little silly. The way that hit points work is unrealistic. The rules for buying, selling, and crafting items are nonsensical, in that it's unclear what sort of economy could conceivably support them. And so on. Verisimilitude in D&D is achieved by not thinking about things too much, not through attention to detail.

All of the complexity of the system isn't there to create balance or realism; it's there to give geeks something to fiddle with endlessly, because we frankly like doing that.