Because ransomware criminals would probably love to take over that site and start distributing ransomware or other malware from it. And I have no way to judge how well that site's owners have protected it from that sort of attack. When I'm in doubt, I tend to err on the side of caution.

I assume most people reading the website would be those already infected.

The criminals are more likely to take over any other high traffic website and infect computers of clueless people who have never heard of ransomware.