In game theory, if you start with more capital, you'll - practically speaking - win out every time. So to avoid an entrenched class system that will widen overtime, tax inheritance allows the country to not be lord over by a group of generational billionaires/trillionaires that you can't get rid off. That's in theory. In practice those you'd want to prevent from lording over you can hide their money and continue to lord over you for ever.