Part of this is that we are increasingly in self-selected communities of people just like us. Prior to the Internet and social media, you more often interacted with people that all you had in common with was spatial location and a dash of socio-economic status. It wasn't an unbiased slice of the populace, but it was at least less biased.

But today, it's much easier to have all of your social interactions limited to a social media bubble that reflects yourself.

That in turn makes it really easy to believe that whatever is true for you must be true for everyone because it seems to be largely true for all the people you see on a daily basis.