Because some people want more than the equal share that they amongst all others on this planet actually deserve. (-The term 'deserve' in this context refers to John Rawls's concept of 'desert')
This is one important reason why the issues of distributive justice are such an urgent matter to be tackled and to be rectified, so that poverty can be eradicated and all human beings can develop their potential to the full. This would also entail that they come to reason that it makes no sense to treat each other as competitors (or even enemies) and to ride roughshod over the world.
We must learn to live together as brothers in sisters or perish together as fools (Martin Luther King 1929 - 1968)
For further reading on poverty I warmly recommend the pamphlet 'Documents on Poverty, Race & Globalization by the Women's International Coalition for Poverty to you.

rideOn,
bert