Because testing everything is not feasible.
Why are there so many software bugs?
Give me five solutions to problems that occur during software development
What makes a good test engineer?
What is Software requirements?
What is a test plan?
What is a test case?
What should be done after a bug is found?
What is configuration management?
What if the software is so buggy it can�t be tested at all?
What if the project isn�t big enough to justify extensive testing?