Fix it? Sure, except despite their ostensibly open nature, many of these projects are run as internal projects of large tech corps, and the bandwidth and interest of the team in reviewing and merging external PRs is limited to nonexistent. I've submitted small, obvious bug fixes, complete with test coverage, to projects and had them sit unreviewed for years.
Fork it? Well there is a big, big difference between spending an hour fixing a bug and submitting a PR, and forking a large and complex dependency. You're committing yourself and your team and employer to the ongoing maintenance of the fork and merging of upstream changes, just so you can merge a bug fix. Usually, that's just not practical.
Fuck off? Many third-party dependencies in mature projects are non-negotiable. Removing them would require a complete rewrite. You may not even have been the person who chose them, but you're stuck with them, and then your boss asks you to implement a feature or fix something that relies on a change or fix in a dependency. Being blocked due to upstream intransigence is very frustrating, and there is often a professional cost to saying "no" to a request, especially a bug fix, no matter how convincing you argue that it's not your fault or under your control.