> Given that this is common knowledge, why do cover-ups keep happening?
Because the cover-ups work, and only happen when the people involved are deeply involved in the original scandal in a way which would get them in more trouble if it came out than their involvement in the cover up would. The fact that often their role in the cover up does get them in trouble doesn't change that the cover-up is usually successful, even in those cases, at preventing their role in encouraging (or more) the original wrongdoing from coming to light.
Because the cover-ups work, and only happen when the people involved are deeply involved in the original scandal in a way which would get them in more trouble if it came out than their involvement in the cover up would. The fact that often their role in the cover up does get them in trouble doesn't change that the cover-up is usually successful, even in those cases, at preventing their role in encouraging (or more) the original wrongdoing from coming to light.