People tend to build internal hierarchies and create structure that help them understand the world around them, and sometimes, the ideas that we internalize can do more harm than good. For example, many people have an idea that certain crimes are much worse than others, and that personal belief leads them to underestimate the importance of criminal charges they consider minor.
Getting arrested and facing criminal charges has a way of impacting your life for more than just the time you spend in jail. Many people don't want to deal with the interruption that court can cause in their lives. Even if they know they did not commit the crime of which they are accused, they may see it as easier and smarter to just plead guilty.