Well, in English, at least, that's just not true. Many great science fiction writers are arts majors: Kim Stanley Robinson is an example; he has a Ph.D. in English literature. There are SOME have science degrees, yes, but many more who do not. I worked for eight years as a science and technology journalist after university, and I study science myself all the time. It's like being a sports journalist: you don't have to be an athlete to write entertainingly about sports, and you don't have to be a scientist to write entertainingly about science.