Define "hold capital". Most capital held in banks ends up helping the velocity of money because it's used for loans. If we tax the hell out of money stored in banks, banks will have less reserve, which reduces the fraction of which they can lend, which means your average joe has less access to loans for their cars, maybe their first house, a small business, etc.

Past that, if it's expensive to hold money in America, they'll stop holding money in America, which means capital flight, which is its own very large problem worth avoiding.