Tanks get good through experience, you don't get that on a training dummy. The lower level dungeons are the way they are so they can get used to the role so that by the time it is key for them to do it "for reals" they're prepared. Not everyone has been playing MMOs for years or has multiple 50s, there are still people playing for the first time or playing a tank for the first time.

And people wonder why so many people don't know their roles when that attitude is out there.