Growing up, we tend overlook the fact that there are other things to take note of and only accept 'Reality' like a gift that we are given. We grow up to be self-centred and selfish beings where our world revolves around us and only us. We make our assumptions on reality based on our 5 senses. If we can see it, hear it, touch it, taste it and smell it, it exists, it is real and it becomes a part of our world. When it is something that humans are unable to comprehend, we deny it in our minds and ignore the fact that it is there at all.
The chief argument deployed by realists is that we should simply assume that the world is independent of us. If we were to assume everything as they are, how do we know what really is and what really isn't? How do we know whether we are dreaming or not when we only assume that we are awake or asleep? If we wanted to gain knowledge, how can we know anything when all we do is assume? To know of something is truly a complex idea. You cannot know something unless you are able to understand the piece of information as truth or fact.