It may just be that you aren't used to it yet. If you don't read a lot, a large part of your brain is likely being used to decipher and understand the actual physical symbols printed on the page. You might not have enough brain power left to visualize the scenes and characters in your mind.

Sorry if that sounds mean, it has nothing to do with intelligence. It has to do with practice. Like when you first learned how to drive, you likely weren't staring out the window looking at the scenery. You were focused on making the car get to where you wanted it to. But after a few years of driving, I'd imagine you are able to drive without thinking much about it at all. And your brain is able to focus on the scenery, or the radio, because it isn't working very hard to drive the car.

Start with some young adult fiction. The Hobbit, Narnia, The Edge Chronicles, stuff like that. They have larger print, as well as some illustrations that might help you visualize the scenes. Plus they are really great stories!