Because:
1) We live in a closed system (the Earth, you know, being spheroid).
2) Some of us are better at exploiting the system—or “creating wealth”, if you want to be all “cup half full” about it—than others.
3) Those who are better at exploitation—ahem—wealth creation…don’t really feel the need to share. Think you’re immune? Well, are you planning your honeymoon in Zimbabwe?
4) Those who have tried sharing (i.e., communitarians, communists, cultists) end up engendering extreme power polarization (i.e., oligarchy, despotism) even more efficiently and obviously than the “democratic” republic.
5) Because the exploiters/creator$ haven’t figured out how to make money from housing the homeless, making prison populations useful or how to get to orbit cheaply enough to begin commercial exploitation/creation$ in space. Blue Ocean thinking takes imagination, a commodity that is discouraged and rare thanks to our 18th century system and habit of buying up/subsequently sitting on patents.