1. Some philosophers have a descriptive ethic (this is what people do) rather than a prescriptive ethic (this is what people should do). This is certainly interesting, but it isn't what most people mean when they talk about ethics--it doesn't give us any guidance on how to behave ethically.
2. Some philosophers try to work backward to a prescriptive ethical principle from a set of ethical prescriptions which are widely accepted but, importantly, unproven. For example one might start from the prescriptions "murder and lying are wrong, giving and learning are right" and attempt to come up with a prescriptive ethic that unites these prescriptions. But "murder and lying are wrong, giving and learning are right" aren't objective facts even though they're widely accepted, so any prescriptive ethic that comes from these prescriptions can't be objective either. It's not coincidence, for example, that in the west, the atheist/moral realist philosophers mysteriously often start from Judeo-Christian prescriptions.