Common fault on most cars - dirt/grit gets stuck in the window scrapers/seals and drags up/down the glass causing noticable scratches over the years.
Just one of those things. No real easy solution other than replacing the glass and cleaning/replacing the seals.
You can polish some scratches on glass but I'd wager that it'd be too much effort and not that effective for the side windows unless you're really OCD!
You might be able to polish them with the right sort of kit - machine polisher and compound etc. There's loads of companies out there that do this sort of work for shop fronts in city centres etc that get scratched by kids.
Being realistic though, I don't think most people notice them. Only once in a while whilst detailing do they get noticed but if they're really bad I guess replacing the glass might be worthwhile