Because if not, people with insatiable appetites will simply seek control of those corporations rather than direct control of piles of money.
Personally I'm an advocate of the exact opposite of what seems to be proposed here: controls on corporate size rather than individual wealth. Living cells don't grow without bounds; at a certain point they get too big and are forced to divide. Simply imposing a tax on corporate size (not a limit) would accomplish this.
Industries where massive size is simply unavoidable (e.g. passenger airplane manufacturing) would simply pay the tax; any economic distortions can be nulled out by reducing the industry's VAT rate in proportion to average size-tax paid by that industry. In all other industries smaller competitors would have the persistent advantage of lower tax costs, allowing them to eventually undermine the bloated colossuses.