Earlier quoted context omitted.
Your assertion ignores the "need to know" aspect of classified information. Just having a clearance does not give you permission to access arbitrary secret data any more than having a driver's license lets you drive backwards down a highway.
That's nice, but obviously there was no oversight, no logging, not even simple rate monitoring. That is, after all, why Manning was able to wget this massive cache of data, and they would be none the wiser had he not revealed his involvement to people or systems he falsely deemed trustworthy.
no oversight – how about "insufficient oversight"
no logging – I doubt this is true
…rate monitoring – This gets into a huge, hairy, and probably unsolvable problem called "aggregation". Briefly, I might be allowed access to specific bits of classified information in order to do my job, and each one of these is within my clearance. But, if I were to collect together enough of these facts the collection as a whole would provide more information than the sum of its parts and would need to carry a higher classification marking. Consider, people working at an airport need to know when a cargo plane is going to take off or land, say to plow the snow off the runway. But if I take all the take off and landing data, now I know the material flow, and if I take it over time, I can see what the military is strategically planning. This is the kind of problem that is so complicated that after coming up and derailing a few design meetings the officers in charge will realize that the "pie in the sky" ideas from the contractor will probably cost more than the whole rest of the development and never provide a usable solution, so it gets shelved for later releases and some sort of fig leaf is put in place to cover the mandatory requirement.
… able to wget this massive cache of data – I haven't looked at this specifically, but speaking from similar systems… There may be entities that access the data at high speed, especially reporting and analysis systems. (screen scraping is frighteningly common as a way to interconnect systems in the military) He may not have been an obvious outlier. Or maybe no one was watching. Or maybe he used wget's rate limiting options.
…they would be none the wiser had he not… – This is where the auditing comes in. After fact A,Q, and U are released, someone would probably start processing logs and see who retrieved A, Q, and U. Manning's indiscriminate collection would make him likely the only person that retrieved the union, or at least the only human on a list with a bunch of automated systems. It is after the fact, and too late to prevent damage, but it is possible the auditing would have brought him to the attention of investigators.