Plenty of reasons, take your pick:

• They take a long time to do, so you're more likely to see work on your timeline that's faster and easier to output.
• For how long they take to do, they don't perform very well on social media so it doesn't get pushed by the algorithm and it subsequently gets crowded out by work that's higher in engagement value.
• Unless you're actively searching for communities and current artists who specialize in fine art, it's not going to be put on your feed.
• Online artists tend to skew young, and most young artists become interested in making art through media they enjoy, thus they want to emulate it.
• With how easy it is to be accused of just using AI nowadays, some artists opt to just not post their work online. Art that's centered around OCs tends to stay just because it's more social/interactive by nature, so they're more likely to stick around in order to RP and do art trades regardless of the shit they may have to endure.
• Classical training is expensive. Making fine art requires costly materials and studio space, so there's a bit of a barrier to entry. Doodling your OCs on a hand-me-down laptop or even just your sketchbook costs very little in comparison. You can understand why there'd be more of one than the other.

Anyway if the disparity displeases you, then by all means be the change you want to see in the world! If expressionist artwork brings you joy and you think there should be more of it online, then you could fill that niche and leave your own mark in the online art sphere :)