Like most of humanity's positive and negative traits, this is a result of our cognitive limitations. You have no doubt run across the concept of the monkeysphere (it overlaps significantly with the content of your post) which is essentially the maximum number of fellow humans we are able to give a crap about. Anything above that gets really really ... diffuse.
Moreover, something that gets diffuse under those circumstances? Responsibility. There was this quote floating about on WhatsApp a few years ago; something about how Everybody assumed Somebody would do it so Nobody did it and Everybody suffered for it. Something like that.
When it's just you, Character Becomes Who You Are In The Dark. When there's just a FEW of you, you know you HAVE to do something (if for no other reason than having witnesses.) When there's a million of you though? And you're late for work at your boxed-in cubicle in your boxed-in skycraper, Starbucks coffee in hand? Ignoring a child hit by a car starts to look like a sane reaction. "After all, someone else will handle it ..."