Why in the world do people do this?

I've seen it countless times for most illegal niches and it's absolutely bothering me as to why it's being done for almost every (DOWNLOAD / STREAM) niche.

Why do users create posts on websites like VK, Wattpad, various other forums, and promote them in the form of a lander appose to an actual website?

It seems like they just create a bunch of spammy links, for high target keywords on authority websites and then manually crawl the pages in hopes that it indexes in google and sometimes the method works.. is there a more logical reasoning behind this?

I think you may have hit the nail on the head as to why they do it in your post.

"sometimes the method works"