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
As to your example, you won't pay a wealth tax on your car if you increase its value under Zakat. Now if you want to use it as a currency (start flipping cars) then it's different.
The problem is the rich have lots of options so you need to consider second order effects. If you print lots of money (as we are now), people and especially the rich tend to exchange their dollars for assets, such as stocks, art, etc. If you attempt to tax those assets as well, people will purchase other untaxed assets.
I'm also not sure it actually matters as much as people think. It's not like Gates, Bezos, Zuckerberg, or Musk are using their funds to compete with the average person, IE buy up all the beef. Them having less really won't make us more efficient at producing food or increase the amount of housing available in cities.
I would argue that drastically cutting inflation might help reduce income inequality over time.