1. The numerical order is usually arbitrary, so they might as well have been bullets.
2. They often leave out important points in order to reach a particular number like 5
3. There is no 3
4. They pander to a lazy reader who can't absorb prose, but as people become more accustomed to reading they will become more comfortable with reading longer pieces of text.
5. They add useless items just to pad up the list to the number they wish to reflect.