two very different games fundamentally
No they are not. Sure, they have different art styles, some different mechanics, but they are the two largest Battle Royale games out right now. The only games I would consider to be closer to PUBG than Fortnite are perhaps H1Z1 and the Arma mod (that PUBG devs made).However, as for:
Why does it need to be a zero sum game with one winner
It doesn't, but I am so thoroughly frustrated with the PUGB developers that I hope they suffer financially enough to do something about their poor development practices.