Coz it's a very different job trying to find something when a) you don't know specifically what you are looking for, or b) you are unable to access the documentation that might show you where to look (or what happened).
If somebody opens their mouth and says 'Sponsor X is paying player Y through mechanism Z' you at least know what information / documents to attempt to obtain.
It's easy enough to audit what is actually disclosed. It is much more difficult to audit what is not disclosed and what may not directly involved the club. Let's say Sponsor X does pay player Y in cash at a golf day, Or Non-sponsor (but 'friend') J pays player Y by paying them to do Z. Even if the auditor somehow got wind of this behaviour, he can't compel the player and 'friend' to play ball. If they say 'no such event / payment occurred', it is not easy for the auditor to establish that it did.