As someone who wants to contribute to a project for the first time, there would be nothing more frustrating than spending time creating what I think is a good pull request, only to have it be turned down (which it will be).
Issues help new contributors and repo veterans determine whether it's worth the extra effort to create a pull request. The problem with low-quality issues needs to be addressed by automation and filters, which GitHub hasn't done yet.
If this is a random one-off drive by bug, then why go through the hassle of figuring out what particular standards any individual maintainer wants to follow just to have it turned down, or discover that some regular contributor already patched it while I reading the documentation.
(I mean, I'd love to contribute, but the few times that I've stumbled upon a bug, fixed it, figured everything the maintainer wanted in a pull request... it was aleady fixed by the maintainer and I'd just wasted a lot of time)
that's not a very scalable thing to do, as it involves unbounded amount of work. what if the maintainer never pull in your fix?