The operating system isn't needed for that at all and blaming the vendors is fundamentally misplaced. That directly sending to other computers via direct IP connections is technically possible if you don't operate any firewalls or similiar to block it. And of course non-fixed and shared IP addresses would complicate the system.

You would find out quickly why you don't actually want that when your own "unisystem" designated port server/client gets hammered with unsolicited requests and exploit attempts. Easy is a matter of interfaces although other design choices would come with costs. Safe however would be far harder. If you set it to a whitelist well you lose discoverability instantly.