Why do we have this? Because people suck and it just makes way too much money for anyone not to do it. Why didn't we have this 20 years? Because the technical capability wasn't there.
It's really no different here. Communication and messaging costs have really gone down to zero. If it wasn't FB, it'd be someone else. There's simply too much money with very little costs in engagement bait, whether or not that's the intent of the platform or product.
And yeah, that's the case because people suck. Most people aren't looking for verifiable information. They're looking for whatever or whoever says whatever it is they've already chosen to believe. That's it.
I'd say the biggest problem with FB and Twitter is sharing links as this is such an easy way for the lazy, ignorant and stupid to signals their preconceived notions to whatever audience they happen to have. But if Twitter or FB didn't allow sharing links, someone else would and that someone else would be more popular.
I honestly don't know what the solution to this is.