Invented by 3Com founder Bob Metcalf: https://en.wikipedia.org/wiki/Robert_Metcalfe

Because it works exactly for the thing it was designed to do, and does it well. Like asking why we still use hammers to pound nails in after a thousand years. Some things are going to stick around for a long while and not just go away because “progress”. I fully expect some form of Ethernet to still be used 50-100 years from now just like analog phone lines are still used now, and those are over 100 years old.