Sometimes I think these global comparisons are like comparing different planets. Who really cares? It's our immediate surroundings that matter.
This means that inequality is important for social stability - does social stability matter?
There are possibly other reasons as well - you could argue rich people invest differently, which has implications for the global allocation of resources. [2]
[1] http://science.sciencemag.org/content/318/5854/1305.full?ijk...
[2] https://www.theglobeandmail.com/investing/globe-wealth/artic...
How the rich on earth behaves does affect poor people: It affects what resources are available to others, and it affects prices, job opportunities, investment in the immediate surroundings of poor people, and can do so both positively and negatively.
Your analogy is flawed, unless you think things like progressive taxation, social support / transfer payments, and closing rich’s tax havens and loopholed are equivalent to a galaxy far far away.