Because a world in which many parties have strategic nuclear defense AND sufficient non-nuclear forces is more stable and less conflict prone than a world in which major powers possess only nuclear weapons.
Such was the worry about Cold War Germany: had Russia attacked with conventional forces, the US would have been all but required to escalate quickly to nuclear weapons.
Similarly in Korea. If the Inchon landings hadn't been successful, it's a coin flip as to whether the US would have nuked North Korea, Russia, and/or China. Truman, LeMay, and MacArthur were all prepared to go to the mat for South Korea and Taiwan.
https://www.airspacemag.com/military-aviation/how-korean-war...