It is a guardrail against burnout and tracker spam
Its based on their implied perspective that the majority of submissions don't follow those guidelines which helps determine their quality threshold.
https://github.com/badlogic/pi-mono/blob/main/CONTRIBUTING.m...
If all open issues are actionable items, that makes expected workload a lot easier to handle.
If most open issues are actually in "needs triage / needs review" state, you lose the signal from the noise.
The issue tracker for a project exists primarily as a tool for maintainers, not for outsiders. Yes, the maintainers could change their workflow to create a new view that only shows triaged tickets.
Or, they could ensure the default 'open' view serves their needs.