I have a secure computer.
A Commodore 64 that's gathering dust in a box. It's not plugged in to power, has no internet connection, doesn't run any modern OS, can't run most modern programs, is impervious to all modern exploits, and any successfull hijack would vanish with a simple reboot.
Granted, it also can't do much in the way of computational grunt work, but that's not what you said. You said there was no single computer security. That view is false. There is such a system, it's just that it's also not up to the task for which you need a massive supercomputer.
But I'll happily challenge you to a game of Blue Meanies from outer space! =-)