Why do you think a general-purpose computer can't be built which is equivalent to a human brain?

In order for machine inteligence to threaten human beings you are going to need a lot more than just a smart machine. You need many, many systems and more factors than just speed.

The human brain can take advantage of a lot of interactions that computers do not take advantage of, and as a result gets a lot more efficiency while doing what it does. More processes with less power. Consider that it is true for nearly all things people do, where a human being can survive longer than a machine in an environment, and I figure that machines will never threaten regular, carbon based, life.

Not strictly true. Evolution is about adaptability, not efficiency.

It's about survival. When resources are limited, being able to do more with less is a huge trait that shouldn't be considered lightly.

What computers are made of doesn't really change how they act or behave. If we were to create a computer system that acted like life, used similar materials, and looked like life, we wouldn't be calling it machine intelligence, we would be calling it artificial life. At their core, circuits are still wired up to use the switching of gates to process information, and have tons of inefficiencies in them for the sake of allowing us to design them easily to do one specific task.