Because if you're only reading one email, and not the reply, and the reply to the reply, then you're taking it out of context.
Example:
In an email frequently quoted by deniers, Kevin Trenberth says, "The fact is that we can't account for the lack of warming at the moment and it is a travesty that we can't. The CERES data published in the August BAMS 09 supplement on 2008 shows there should be even more warming: but the data are surely wrong. Our observing system is inadequate."
http://www.eastangliaemails.com/emails.php?eid=1048&filename=1255352257.txt
Apparently you think this email "proves global warming is a scam." If so, you're taking it out of context. Because here's the reply to Trenberth, from Tom Wigley:
"I look at this in two ways. The first is to look at the difference between the observed and expected anthropogenic trend relative to the pdf for unforced variability. The second is to remove ENSO, volcanoes and TSI variations from the observed data.
Both methods show that what we are seeing is not unusual. The second method leaves a significant warming over the past decade.
These sums complement Kevin's energy work.
Kevin says ... "The fact is that we can't account for the lack of
warming at the moment and it is a travesty that we can't". I do not
agree with this."
http://www.eastangliaemails.com/emails.php?eid=1051&filename=1255496484.txt
So Kevin Trenberth says we can't account for the lack of warming, and Wigley immediately shows that we CAN indeed account for the lack of warming, from solar (TSI) and El Niño (ENSO) effects. And what is Trenberth's reply to this? Does he stick doggedly to his guns like a denier, unaffected by actual evidence?
Nope. You've misunderstood my point, says Trenberth:
"Saying it is natural variability is not an explanation. What are the physical processes? Where did the heat go? We know there is a build up of ocean heat prior to El Nino, and a discharge (and sfc T warming) during late stages of El Nino, but is the observing system sufficient to track it? Quite aside from the changes in the ocean, we know there are major changes in the storm tracks and teleconnections with ENSO, and there is a LOT more rain on land during La Nina (more drought in El Nino), so how does the albedo change overall (changes in cloud)? At the very least the extra rain on land means a lot more heat goes into evaporation rather than raising temperatures, and so that keeps land temps down: and should generate cloud. But the resulting evaporative cooling means the heat goes into atmosphere and should be radiated to space: so we should be able to track it with CERES data. The CERES data are unfortunately wonting and so too are the cloud data. The ocean data are also lacking although some of that may be related to the ocean current changes and burying heat at depth where it is not picked up. If it is sequestered at depth then it comes back to haunt us later and so we should know about it."
http://www.eastangliaemails.com/emails.php?eid=1052&filename=1255523796.txt
So when Kevin Trenberth said "we can't account" for the lack of warming, he didn't mean we don't know the ultimate cause of the lack of warming -- since that was obviously El Niño -- but rather, that we don't know the immediate cause: where did the heat go? And its clear from reading both emails IN CONTEXT that what he's complaining about is that the global climate isn't monitored closely enough to answer that question.
That's what I mean by "taking out of context." Trenberth's emails don't prove any kind of hoax, in fact they prove the reverse: good scientists working hard to find the truth and pin down the details.