The reason for this is The Algorithm. Facebook harshly limits the reach of any page that posts links to any website, so the only way for a page to reach users and convert that audience, making their followers visit their website, is to avoid posting links altogether.
The algorithm seems to penalize link posts particularly harsh. You may notice that most posts on Facebook on any hashtag are never link posts but vertical video posts. For example, you would imagine that the hashtag #news would be full of links to news articles, but it's actually full of videos. This is true through all of Facebook these days. Everything is just vertical video, never links, and even images are rarely seen.
Consequently, social media marketers avoid link posts. Sometimes they will post an image post with a link in the description. Sometimes they will say you can find a link in the bio (their profile page) to avoid posting a link at all. And sometimes they will post the link as a comment and tell people to check the comments for the link. It does feel strange that these practices work when link posts are restricted, but they actually do for some reason. I guess that's just how Facebook works.