If you're contemplating a question, it's "Why DO balls have a stitch line?" (mind the question mark). If you're writing a body horror fanzine or medical blog post clickbait headline, it's "Why balls have a stitch line". In the latter case, readers will expect your content to answer the (indirect) teaser question.

Because that is where the vagina would split in embryonal development. Ask AI, it can tell youXD

Why r there no balls in this picture I feel bamboozled

Vasectomy scar