The way VPN clients are marketed; not the “killswitch” itself.
Security is usually a shared responsibility. If there’s a “killswitch” then a client is better off using it, because if it doesn’t, all bets are off. The traffic may then leak not just because of the OS’ shortcomings but also because of the VPN client’s. The latter is in the control of the VPN provider, the former is not.
For example, VM & sandbox escapes do exist (due to bugs in the implementation or bugs in the OS/Kernel); but that doesn’t mean projects like Whonix/Chrome put the towel in and abandon isolation/sandboxing. Those projects must continue to use the tools made available to them by the OSes and sandbox/isolate to the extent feasible. Without them doing that, all bets are off.
You just said Whonix does? I am not sure what your stance really is.
Like mentioned above, this is offtopic for this thread. Do consider contributing here: Mention kill switch leaks caused by OS limitations