Abuses of power (of which wealth can be used to acquire it) aside[1]. The main arguments that allow inequality are arguments against the costs of enacting outcome leveling policies.
Some examples.
- The growth rate of an economy which enacts redistribution policies will be stymied, leading to lesser outcomes for all, particularly in aggregate, but over the long run also for the bottom percentiles. The rough idea is in absolute terms now is one of the best times to be in the bottom quartile basically ever. This is measured in things like total consumption, access to medicines and technology, number of TVs and airconditioners etc. By changing the growth trajectory through redistribution today we improve the lives of todays people at the cost of lowering the lives of future people who would have had their lives lifted by higher amounts of growth over time.
- Redistribution as a form of injustice -- property rights are an important element of a functioning non-authoritarian society, and taking from someone in an unjust (using that word beyond "legal" or "democratic"-- remember the tyranny of majority issue) represents a degradation of society of sorts. There is little difference, in many folks minds, between a mob with pitchforks versus a mob with ballots. The cost of redistribution might be less "justice" in a system. This deters people from viewing the state (city, region, country) as an investible market.
- So long as wealth is able to generate wealth (such as owning an income producing asset) inequality will be inherent to reality. The compounding effect of purchasing assets with asset derived income will accelerate the gap in individual earnings. Making failing bets (hence why gov't bailouts are really bad), and the regression towards mean outcomes (hence why maximizing the bottom half's opportunity is critical), are crucial to the system. And in the US we've essentially done the opposite through bailouts, only hiring from expensive universities, denying certain investment classes only to the elite accredited folks, and by erecting regulatory barriers to entry that disproportionately encumber the little guy in the market.
- Assets buying assets results in increased average price of a dollar of earnings which creates net opportunities for those who transition from employee to entrepreneur. One way to see historic S&P p/e multiples is to see it as the reward for creating a dollar of income stream as being highly compensated today, whereas it was less compensated in the past.
[1]- Particularly citizen's united in the USA context, and any similar issues in other countries