This is not true if you consider security-in-depth. Many of them are exploitable on their own but maybe not in combination with other issues that are as yet unknown or known but not patched everywhere.

As a simple example a local privilege escalation issue that is not exploitable on a device that only I ever have access to, essentially becomes a remote root access flaw if you have untrusted or unreliable users (clients with accounts for instance) on the system. This works on a finer grain too, seemingly minor issues spread through the kernel and user space can add up to a serious exploit.