People don't read. That's pretty much it. Maybe, maybe, if there was some in-line text above the issue form submission, and it was very short (like one or two sentences), people would read that. But any long-form prose is going to be skipped, and likely something that requires clicking a link to read an external CONTRIBUTING document isn't going to be read either.
I ran into this a lot in pre-GitHub (and pre-git) days when maintaining OSS. It got to the point where I would reply to emails with a simple one liner "Addressed here: [link]" or, "Bug report requirements: [link]". I felt horribly rude doing so sometimes, but I realized that people were not respecting my time by failing to take a minute or two to read through the bug report guidelines, and so I wasn't going to waste my time being polite. There's unfortunately a lot of entitlement among open source users, and it gets tiring after a while.
I ran into this a lot in pre-GitHub (and pre-git) days when maintaining OSS. It got to the point where I would reply to emails with a simple one liner "Addressed here: [link]" or, "Bug report requirements: [link]". I felt horribly rude doing so sometimes, but I realized that people were not respecting my time by failing to take a minute or two to read through the bug report guidelines, and so I wasn't going to waste my time being polite. There's unfortunately a lot of entitlement among open source users, and it gets tiring after a while.