Why are computers one of the biggest waste issues facing the world?

I assume that when you say computer waste, you refer to the literal waste of computer and electronics components. 
Computers and other electronics equipment may have a certain lifespan once built; most computers are built to last three to five years, but in the right conditions, computers like those aboard satellites and submarines can last for decades in usable order. At the same time, however, computers can become obsolete. This may happen faster than the computer itself degrades, and can result in computer disposal after a short period of time.
What happens to disposed computer waste depends. Obviously, most is simply thrown into the trash, where the spacious inner compartments are crushed to reduce space. Some get recycled, as is more the trend these days, where silicon, zinc, copper, and precious metals like gold can be extracted, while plastics and steels are reworked. A tiny percentage is actually repaired and reused. Some enthusiasts are using oil baths and similar methods to run archaic computers long after they are considered obsolete.
If you are referring to the waste of computer power, such as computers sitting on, connected to the internet, and running no processes, there are many solutions to this problem. Some people have begun lending their computer's spare power to research institutions, so that with thousands of people's computers running a job in the background, a virtual supercomputer can be used. Other devices, such as smart readers on water mains, are connected to the internet as well, but are unused for almost the entirety of their lives. Some companies intend to sell the spare computational power of their microcomputer grids as supercomputers as well. In general, it could be said that the world has far more computational power than it needs at any given moment.