Again, this is a different type of spam. They're not looking for clicks. In fact, they probably don't want you to click the links.
They just want webcrawlers to SEE the links. That's why they go back and secretly add them weeks later, long after anyone is going to see the post. If they wanted clicks, they would try to put the links right up front in the begining.
It's all part of
SEO, Search Engine Optimization. The webcrawlers see the links in many locations, and that triggers positive action towards that website in the search engines.
Also, just because "Jesse" replied, or added something more complex than a link doesn't mean that "Jesse" still isn't basically a search and replace algorithm. Someone might sit and type out dozens of replies to dozens of websites, but it's still an automated bot that goes and does the text replacement. They use these forums because the same forum software is used over and over again, so the bot programing can be repeated over and over again. The background code is the same across dozens or hundreds of websites, which makes multiple attacks easy.
*Not an internet safely expert, but yadda yadda Holiday Inn yadda yadda.