It kinda does
Let's see, let's see...I'm just grabbing titles here, so I can't vouch for the quality of any of these comics:
Drama: Warning Label, Caster, Late Bloomer
Thriller: Witch Creek, Finality, Elena
Sports: Midnight Furies
Romance: Humor Me, Lord Have Mercy, Circuits and Veins
Sci-fi: Fatherboard, Cubicles, Our Universe
Still, the lesson is, you just have to look. ^^;
They ARE harder to find, but I think if anyone's to blame for that it's the Western animation industry, that made the assumption that teenagers don't watch cartoons. So, the majority of serious, story-driven narratives in the 2D art form that young adults enjoy come from Japan and other parts of Asia, and have done so for the past few decades.
So it follows that when those young adults want to start their own serious, story-driven, but still YA-targeted webcomics, they gravitate towards the art styles in which most of them are found.
It's a tough wall to break down, although some more recent Western shows (like SU, Gravity Falls, Star vs, etc.) that aren't afraid to take serious directions and actually are produced with an older fanbase in mind are helping, but an environment in which a Western equivalent of, say, Attack on Titan could be born is still pretty far off.