If "evolved naturally" isn't dramatic, why is it that they can get away with having this plain undramatic origins but yet the Borg, in your opinion, cannot?
I'm pretty sure that I can guess at this one, because they never spend an entire episode just sitting around and watching primordial ooze bubble or lesser apes crawling along the ground. Because billions of years of natural evolution isn't worth dedicating an entire episode to, or an entire book.
I apologise in advance for what will probably be a very poorly articulated argument.
I would have thought the nature of the Borg is what would dramatically prevent them from just being another 'naturally' evolved race. We can easily imagine a race like the Klingons (aggresive, war mongering) or say the Cardassians (racist, smug, ruthless) since these are things that we can easily see in ourselves (a naturally evolved race), so a radial origin story isn't required.
The Borg on the other hand are much more...primal force. It's so far out of what most of us can percieve as happening to any species like us naturally. The total surrender of individuality, the enslavement to a singular will, completely alien for most of us, to a force that has a singular motivation that most of us would neve conciously consider.
Again, that's probably not really described well what I'm trying to explain here, but hopefully something will sink in there....