One important purpose of requirements is collaboration. This is why all modern requirements tools are built with collaboration features that enable seamless communication in the context of requirements. And before that, collaboration was done by exchanging documents.
Organizations Stay Largely Isolated
Using a requirements tool internally for collaboration is fine, as everybody is on the same network. But for external stakeholders, this is rarely feasible due to concerns about security. Using the same tools would require your tool to be accessible on the Internet, or at least shared with collaborators via a secure connection (e.g., a VPN). Even if this is possible, decision-makers may have concerns — justified or not — about IT security.