Thanks for the feedback on this! I’d love to provide some more context.
Making someone a member of your organization comes with permissions and privacy considerations. Because of this, it may be safer and easier to make whatever repositories you’d like the general public to be able to contribute to public. This way, GitHub users that aren’t members of your organization can still contribute to your organization’s projects.
That said, I also understand that some organizations may not have the privacy concerns that others may, and therefore, adding new members to the organization may be a non-issue. I can definitely see the value in the ability for requests for membership in those situations (this could also reduce the workload for admins, having to remember to proactively add new members). I’ll definitely share this feedback with the team!