The reason your tax rate should be determined by your income, rather than the first integration of your income (i.e. wealth), is because just having wealth isn't hurting anyone. If I spend every evening in my garage fixing up an old car, and increase its value from $1000 to $2000, I've increased my wealth but I haven't negatively (or positively) affected anyone else. It might be useful as a means of generating government revenue but it isn't something that needs to be taxed on its own.

Spending money, however, does affect other people - it reorganizes their incentives to produce more of the thing you spent money on. This can lead to weird situations where a homeless person has 0 effect on market incentives even though all they want is a blanket, but a rich person can motivate tens of thousands of people to assemble her another yacht to leave at her vacation home. So you can make your car aa valueble as you like in the privacy of your own home, but as soon as you want to use that to start affecting market incentives (by selling it then spending the money), prepare to get taxed.

Technically a consumption tax would be better than an income tax, but most people spend most of their income so it isn't that different and it's easier to make an income tax progressive