The point is that auditing involves a lot of detailed work. The weaker internal control is, the more detailed work is necessary. Generally, most of the detailed work is carried out on a test basis. Tests, of course, are all very well if adequate internal control ensures confidence that a relatively small sample really represents the whole.
This confidence exists only to a smaller degree if there is a fair possibility that the figures are biased by undue interference by one or a small number of closely connected employees.