The main reason is that browser-based defaults, namely for text, are decent.
Consider resetting all margins and paddings. From then on, you need to set custom margins for <p>, <hX>, all sorts of lists... when browser defaults make this a lot less of a hassle and, at least in my experience, a lot easier to read. As a bonus it also gets an author experienced with how different browsers perceive different elements by default.
Basically, set only what you need to set. The HTML specification even creates guidelines on how browsers should display certain elements by default.