Why do people buy things and then void the warranty?

I ask only because well, I do. That is to say, some people buy items such as video cards and processors and they have warranties usually from 1 to 3 years. These warranties usually stipulate that overclocking will void your warranty. So why, I ask, do people overclock? I mean in the past even I have been guilty of doing this on cheaper hardware. However I have a pretty new processor and graphics card both with three year warranties. I for one do not have the income to replace hardware if it becomes damaged. Not that I think it will. This is just something that has been bugging me for a while. Why do people do it? Give me a reason, please. I just want to know. I mean I did just to see if I could do it, but like I said that was on older hardware and stuff.