Why does arXiv not allow numbered lines in submissions?
In my experience, it is much more useful to give some context (a few words, name of the theorem, ...) to any editorial suggestions than line numbers. My documents exist as LaTeX source, so line numbers in some printed version don't correlate at all with line numbers in the text I currently work on. A few words of context are easy to search for in a text editor, line numbers would require referring to the printout to find same first. Besides, text changes (paragraphs get rewritten, shuffled around, new chapters are added, a new example appears, ...).