Because you care about privacy and other people knowing you're looking at the job postings?
Because you don't want a man in the middle modifying the job ad to say "must be 7 feet tall to apply"?
And finally because security should be the default unless there is a good reason to do it some other way. It's a bit like having an unpainted car because you're going to drive it in dry places where it won't rust... Unless there is a benefit to an unpainted car, most people take the painted car...
An unsecure http GET could be modified and redirect the interested person to a fake site, even if the original target would do nothing but redirect to its https-variant.
I am not sure why it's relevant whether methods are supported by GUI browsers at all (please refer to URIs by method, not "protocol"); because the security.txt is likely to be parsed automatically (since it is, of course, not HTML) and indeed, "tel:" and "mailto:" are both somewhat apt methods to be invoked by a company who's hiring/receiving reports, and doesn't want/need a website for it.
So yeah, it is important that this part of the RFC specify a difference between "web" and "non-web" URIs, because the authors of security.txt are free to use any URI method that makes sense.
SFTP (sort-of-FTP over SSH) was never browser supported afaik.
I'm not sure about FTPS (FTP over SSL/TLS), did browsers support it when they supported regular FTP?
Or what? What is the point of even specifying this? It's not like any human or bot is going to ignore the link if it starts with http://