I noticed the same thing and I wonder if it's related to the multitasking process of our brains (maybe for some of us?), because when you talk normally, you would be thinking while talking, but when reading, everything is just laid there for you to say. I've been learning Japanese for a while and I definitely don't stutter much when reading even though it's in a relatively new language but when I try to say something, the effort to compose a new sentence definitely makes me stutter a lot, even when being alone.