I remember in the Windows 95/98 days asking my friend for his IP address and then browsing through my windows explorer to his mum's fully shared windows C drive. She had her computer plugged directly into the modem and was using ICS to share the net to my friends computer.
replyI sent them a screenshot of her shared drive through ICQ, and not long after she bought a router.
... because people (by which I mean software vendors who should have known better) irresponsibly failed to create secure systems for those average users to use. A whole lot of which came to be justified by "it'll be behind a firewall" thinking.
replyDid you forget dialup was a thing? No one created anything for home users thinking it would be behind some firewall because as said Windows didn't ship with one, and because the dominant way of customers getting online was dialup, giving a public IP to every user.
replySoftware was created with no security because no one demanded it because no one cared. Technologies that came later did not create that situation.