like book covers for fanfic etc—that shit fuckin boggles me. Like if you aren’t doing this for money or clout, why aren’t you just sharing 100% your own creativity?

Because fanfic authors are not visual artists, so they lack the skill. But probably still want the visual appeal and associations that come with having a (non-stock) book cover.

Isn’t that the reason to share creative stuff, to share a part of your “soul” with others and hope to genuinely touch someone, across time and space?

I think that's getting a bit metaphysical. I think for a lot of people it's just the impulse to share something neat. The mere fact that you think it's neat means you're sharing something of yourself, just not on this soul-bearing level that you're talking about. It's like the difference between sharing a cute cat pic and sharing your true thoughts on your own cat whom you love and cherish.