[zimablue]

It's fundamentally because:

a) most people are along for the ride that it's a moral imperative to keep all of the population happy b) the more money you have, the easier it is to make money. If you don't even that out at all you end up in a feudal society where some dude owns all the land, air, water and you're effectively his slave. Which is broadly the direction America has been going to for the last (30?) years

Fairness is very motivating for people but I don't think it's how you should base your society, I don't really think there's such a thing as fairness as a useful concept.

The real fix is a wealth tax but that is hated by both the old and the rich (look at how mansion tax policy was received in the UK). It's also kind of harder to measure than income.