Basically, the American system simply assumes that people will be self-interested pigs, but through the magical device of checks and balances, no single self-interested pig will gain too much power.
While I’m in favor of checks and balances, I think we’ve seen what a culture of self-interested pigs leads to, and it shows the folly of putting one’s faith in devices while allowing the culture and race to rot. Devices and nifty mechanisms don’t protect you when you have a population of Orcs.