What are you talking about? The UN is involved in peace negotiations constantly.

https://www.un.org/en/our-work/maintain-international-peace-and-security

You might argue that the UN is not super-successful in their peace negotiations. I would at least partially agree. The UN has no standing army to enforce peace resolutions. There is huge disparity in power between countries like the US, China, and Russia and smaller/weaker countries. That also makes it impossible for the UN to enforce peace agreements. That does not mean the UN is "never involved."