Why are there no even-leveled (2, 4, 6, etc) spells in the spell books?

Why are there no even-leveled (2, 4, 6, etc) spells in the spell books?

Why are there no even-leveled (2, 4, 6, etc) spells in the spell books?

I know it seems kind of silly phrased that way, but what I’m looking for is a sort of official, or at least concise, answer.  I’ve gotten the question from a group of people I’m going to be playing with, and while I get that it has to do with spell memorization and moves like prodigy I (obviously) can’t come up with the words to explain it quickly and simply.