Open source is a licensing model, not a community organization model. Collaboration is not a benefit of open source, it's a benefit of collaboration software and a group of people who welcome collaboration. Almost all of the people who like collaborating on software use open source licensing. But there are
plenty of people who use open source licensing who are not interested in collaborating. For example, it is very normal for projects maintained by someone with a narrow focus, or projects with limited or formally organized resources to not accept PRs.
When you send someone a PR, you are demanding that they do work for you to review and merge. Open source licensing does not mandate that they do this. Heck, most open source licenses even disclaim warranty to avoid obligating the authors of even doing work that the law would otherwise require them to do. Now yes, some people will help you with problems. This is because they're nice, not because it's open source.