Because there's no specification for chapter titles in epub. In the default stylesheet I have defined:
Code:
string-set: booktitle "@{@{title}@}@"
string-set: author "@{@{author}@}@"
because those are known by calibre, but the chapter titles depend on how the book is coded, you can add, for instance:
Code:
head title {
string-set: chaptertitle content();
}
to use the content of the <title> tag as "chapter title", but as I say, that depends on the book.