So you think of a pie which is meant to be divided somehow - the winner would get all the pie and the loser none. That might be nice for the winner if they can eat a whole pie but then the rest of us miss out and the winner has to spend way longer at the gym. If you like working out then that's awesome for you. Meanwhile, everyone else is hungry and with the zero-sum game mentality, you probably don't care because you just ate the pie. It's not until the 'winner' needs heart surgery from eating too much that they'll wish they had shared the pie with the heart specialist and the other hospital staff because in the meantime they starved to death.
While you may think you're doing best getting all or most of the pie while others get none, nobody else matters in this equation - it's a superficial win, and bad for you, ultimately, because it's the attitude that'll cause you to suicide.
You see they did research to figure out what made rich white men happy - and then they decided to broaden the research to figure out what made poor (white) men happy. And what they found was that all the money or pie in the universe doesn't make you happy. what makes people happy are their relationships to other people (not pie or money).
Feeling useful and needed is one of the best preventatives against suicide - not eating all the pie. Feeling useful and needed isn't only a preventative against suicide, it is also useful in curbing addictions.
Still you don't have to have the intellect of Einstein to figure out when we're all better off, we're all better off.
While this makes perfect sense to me, even if it is as simple as, if my neighbors kids have opportunities and good income they aren't breaking into my house - they don't need state funded rehab or prison time - all things that my tax has to fund - and even if my tax is funding a great education for my neighbors kids, it means later they'll have good jobs either in other people's businesses or their own and they can finance my retirement. If my neighbors kids are in prison or rehab their prospects will be bad and not great for adding to the economy, other than as cash cows for men who own and operate prisons and rehabs.
Plus they have found that when there is greater economic equality in a country, everyone is better off, and greater inequality is bad for everyone.