What do people think "the rich" do with their money? Store it under a mattress? Keep it in an underground vault so they can go swim in it while laughing maniacally?
Of course they don't. They invest it. They put it in Treasuries, Mutual Funds, Individual Stocks, CDs, Money Markets, Plain Old Checking Accounts.
That money is most definitely not sitting still. It is out there flowing through the economy, financing mortgages, construction, businesses, and probably anything else you can think of.
The only reason to tax the rich as far as economic stimulation goes, is if you believe the Government can do better with the money than the market can. If that is what you believe, fine, just say so and state that as your argument.
But this idea that somehow money is sequestered when owned by rich people is absurd.