Because that leads to a snowballing effect. Money begets money. Over generations, it gets sucked into an ever smaller pool of people. This (among other things) led to serfdom etc in the Middle Ages. We only broke out of that due to the Black Death.

We need a way to allow money to spread back to the general populous. An inheritance tax is a crude, but effective, way to do that.

How would you go about achieving this?