Why? What are they going to do with the money instead? If you charge, say, 5% on unrealised gains, possibly discountable against CGT on sale, say, then do you think rich people are really going to go "well, now I'm only getting 9.5% return instead of 10% return, so I'm going to sell all my shares and build a Scrooge McDuck style swimming pool full of money, instead"?

There are cases where taxes can hurt investment, but you're really talking about quite high rates. For instance, the US peak rate of income tax used to be 94%. That almost certainly _did_ hurt investment.