Why? There are plenty of older useful sites which work just fine over HTTP. If you mean for cases where https is supported, but link is http - I agree.
There are plenty of good reasons to use HTTPS. [0]
It doesn't make sense to link to HTTP when the site works fine over HTTPS, which is the case here. I'm not sure I'd want to completely ban all HTTP though.
Honest question: what is the consequence of visiting an HTTP link rather than HTTPS for a site where my interaction is read only? Is there some security issue? Or is it privacy concerns.
The privacy issue is that your local WiFi provider, direct isp, and all the intermediate isps can see not only which site you visit, but all your activity within that site (like which pages you visit or things you download).
The security part is that any of those who can view can also do a “man in the middle” attack. Comcast could decide to send you a different version of the website that was more favorable to their company, or inject ads (ISPs have been known to inject ads on sites they don’t own before https was big).
A hacker could send you a version that gets you to download malware by replacing content or links. They can see and effect everything you do and see in such a site if they can intercept your request.