You're talking about, at the very least, tens of thousands of lines of code and almost an infinite number of testcases. A lot of it isn't even a result of the game itself but rather the medium of communication; TCP/IP. Lag, desyncs, packet modification and more all open the door for a potential dupe or bug but can't be prevented by code, they can only try to 'trap' as much of it as possible. Catching every single one is basically an impossible task without devoting an extraordinary amount of resources.
Plus there's way more people trying to dupe than there are developers trying to prevent it
They learned from D2 to verify the legitimacy of the packets sent to their servers, but yes, no complex program (especially game) will be completely bug free. And when you have millions of people doing every possible scenario, it is rather impossible to fix them all before release and even then, they can only fix so many per patch cycle.
Most dupes are only around for a little bit, but with the extremely fast ability to share information, once one is discovered, thousands of people can know about it in minutes, thus, the duping is rampant.
there's an interesting read on the legacy of (in)famous D3 dupes:
http://us.battle.net/d3/en/forum/topic/9423113354
my biggest problem with the economy is the rampant nature of gold botting that has been present since pretty much day 1. If the gold you found while playing was actually worth something, there would be a more feasible, and steady (i.e. less RNG), upgrade path for many players.
I hit the 100 million gold picked up achievement a little while back, but even if I had every last "cent" of it available, i couldn't buy an upgrade at this point.
I can't read this, but since it is on the bnet forums, I am going to assume it is 90% filled with people whining about how bad the game is despite them putting a thousand hours into it, 9.99% filled with "nerf barbs omg" crybabies and maybe 1 or 2 posts that are actually intelligent. >_>
And as far as gold pick up, I haven't hit the 100 million gold picked up. I am no where close. But since I played the AH from like day one, I've always ignored gold piles.