Because social media apps are designed to access the same neural pathways as addictive substances. Let me rephrase your question: "why can't you just have a moderate use of alcohol?" Sure, many people can (and note how that's not newsworthy), but try suggesting to an alcoholic to just drink a little less.
This is how I use Facebook. I basically only use it because it's a convenient messaging service; Hell will serve chilled water before my non-tech friends and family join Yet Another Messaging Service™, so I meet them where they already are.
Plus, for better or worse, I rather like the Facebook Messenger interface and app. It's not perfect, but it's nice looking and consistent across platforms. If it weren't for most people I know being on Facebook and not everybody having Apple devices, I'd probably use LINE or stick to iMessage.