For me, it’s because writing markup intended for human beings in a programmatic style doesn’t give me any visual clues as to how it’s going to look. The whitespace is a convenient but only inciental way of achieving that.
Builder is totally fine when I’m outputting formatted data like XML because what little adornment goes into that format isn’t something I need to be conscious of, and in fact, it’s probably simpler if I’m not aware of it at all.