I haven't explained my views at all up until this point, you are quite correct
Ultimately, there are three intertwining principles here that come into play here. Fairness, justice and necessity. On face value you could say that taxing people at different rates is unfair because 'everyone should be treated the same' (a pretty solid definition of 'fair' if there ever was one). But then, not every person has the same opportunities as everyone else, whether that be education, family support, or whatever else. Is that fair? Also, some people also work hard and contribute to society yet do no receive a high wage - is that fair either?
It's pretty clear that few things are life are truly 'fair', but there are things that can justify deviations from 'pure fairness'. For example, if we had a scenario whereby those on higher wages are paying less tax AND those that are earning low amounts (and society as a whole) are suffering as a result, that inequality would be viewed to some (including myself) as a social injustice.
In reality, is is necessary for those that earn more to pay proportionately more tax because otherwise this would could disproportional unfairness to those on lower wages (assuming we have threshold for a decent standard of living and a standard of society we wish to maintain). Ultimately, it is less burdensome and thus less unfair on those with higher wages to pay more tax than the alternatives.
From another point of view, you could argue that it is already truly fair. Everyone that earns a set amount will always be taxed the same. Everyone has an opportunity to earn a wage of any amount, yet they will always be taxed the same amount as an equal earner. Everyone can (within reason) choose their profession and a person can choose a profession that has a higher or lower wage. They will always be taxed the same as the people in the same boat as them.
I am more of an advocate of the former argument, but I think the latter holds some marginal weight.
That's what I think anyway