I think it's mainly because of the false belief that clearnet sites are more dangerous. One thing you should keep in mind is that you should only visit clearnet sites posted on this forum over Tor. I could easily create a site, astorsite.com, and post a link to it here. Maybe it would be a unique URL, and since the robots.txt file blocks search engines, only people on this forum would know about it. Then I look at the server logs for IP addresses accessing that URL. It is trivial to download a list of the exit node IPs and filter them out. Anyone could easily enumerate hundreds of forum users that way. That's why you should only visit clearnet links over Tor. I am surprised when other people are surprised when I point this out. Apparently lots of people copy the link into another browser and visit it directly. I think that's fine for a large site like cnn.com, because it gets tens of thousands of hits to each page, but it's good practice to visit all sites over Tor, that way you don't accidentally mess up.