What's with all the FB spam in comments?

I cannot get through a single article on FB from a media source (the Atlantic, NYT, etc) without seeing either Bernie Sanders spam from Matt Saraceno, or endless movie spam links. You can’t block people in public comment sections so that option doesn’t work for the Bernie bot, and even if you could, movie spammers change names so many times. Does anyone know why FB or the companies can’t do more about this?