Because it's always better to have wealth move around in the population. Finance is like blood, it must flow.

When wealth gets stuck in the hands of a few, it creates recessions and political instability.

In some countries, the poor do not have access to medicine - an injury can bankrupt them, especially as they become unable to work with the injury. Only the brightest may have access to higher education. The purpose of tax is to generally improve the social mobility so that gifted poorer people get a shot at making their own wealth. And also because the improved quality of life reduces things like rich people shootings and arson and other issues from inequality.

I don't think most wealthy people swim in their cash like Scrooge McDuck. They invest their cash in public and private companies which often generates plenty of capital 'flow'.

True, but there's more to it. Interest rate is used to slow GDP (and inflation). Using the blood analogy, it's like lowering the blood pressure. Which means people do freeze their investments or put it into things like gold or crypto.

The other end is it gets invested heavily into profit making stuff. Plantations and farms and power plants are great. But not all of this money benefits a society in the long run. There's stuff like garbage and prisons. There's things that require a lot of investment like educating certain minority groups. Tax money is generally more effective for this. Too much is not good though; we need a balance between state and corporations.