>>what's stopping someone without a proper license to just go and make their own IDP?<<

If someone without a proper driving license from their country of residence did make a fake IDP and manage to rent a car, then that person will really pay the price if they do get into some sort of accident and the authorities begin to review the person's documentation. Probably could get charged with everything from fraud to driving without a valid permit. Just the thought alone of being tried in a foreign country probably works as a deterrent.