Heck, if I buy a piece of bread and there is a dead mouse in it, I can actually return it and get my money back! So many small transactions have safeguards against fraud or fucking up, but handing over the keys to whole nations? Right this way sir, make yourself at home. Imagine hiring a babysitter, and they just eat your baby, clear your fridge, and the cops force you to pay them for their hours regardless. That's kind of where democracy is at.
Similarly, a government that relies on the moral character of its officials rather than institutionalized checks and balances is a weak and easily exploitable system of government.
If you have the checks and balances, you can still hope for politicians that will keep their promises, but when many of them inevitably don't, they will be limited in the amount of damage they can do.