Because in countries with functioning democracies, political power is narrowly scoped (your electors give you a mandate to do certain things, and if you act contrary to those interests you loose your power) and fleeting (you only have power as long as your electors continue to entrust that power to you, and can remove that power if they decide you are no longer fit to wield it).
Money, by contrast, is permanent (capital breeds capital) and unaccountable (you can choose to use the power your wealth grants without any regard for what others think - even if people disapprove, they can’t stop you spending it)