The problem is that you can use wealth and power to chip at (or on a good day take a sledgehammer at) those checks and balances, resulting in an inevitable shift form social democracy to fascism. Checks and balances are addons you layer onto the economic system at work that can change its workings to an extent but not fundamentally alter it; they can't be core to the system due to their very nature. That's why socialists (which, BTW, socialism is a wide category of systems characterized by the lack of private property and accumulation of wealth, not one particular system) call for the complete abolition of capitalism; whether you agree with that or not "but checks and balances" isn't an adequate response.