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In an article, Yes, Forcing an Anaconda to Eat a Human on TV Is a Bad Idea, the "world’s foremost expert on anacondas", Jesús Rivas claims:
"The concern is that all that these shows do is to paint the snakes as the bad guys," he added. "While anacondas might be able to eat a person, this has not been confirmed as a reality, to my knowledge (basically because snake and humans overlap very little). "
Are there no verified cases of humans being eaten by an anaconda?
As of 1999, there were two documented attacks by green anacondas (Eunectes murinus) on humans. Neither human was eaten, see Predatory attacks of green anacondas (Eunectes murinus) on adult human beings, Herptological Natural History. 6(2): 158-160:
In my experience catching anacondas of all sizes, I have found that large individuals are very unlikely to attack when disturbed. Indeed, to the present I have caught and processed more than 120 animals larger than 4 meters and none tried to bite until I (or one of my helpers) had either, dragged the animal out of the water by its tail, or secured a firm grip on the animal’s neck (Rivas 1999). Large individuals tended to swim away when disturbed. Recaptured animals are, if anything, even more skittish than naive ones and try to escape as soon as they detect the proximity of the researchers.
Note the concluding sentence though,
Although anacondas are not "man-eaters" by nature, they are generalists and will take any prey that they can subdue and swallow. Thus, the potential exists for anacondas to prey on people.
Another credible source explains why human consumption by anaconda is unlikely and remains undocumented as of 2012 see section six, Did they ever try and eat you?
Pythons and especially reticulated pythons are a different matter; a 10 year-old was swallowed in 2002 in South Africa, and in the 1970’s a Portugese soldier was found inside the stomach of a python.
Snakes need to categorize their prey visually into "swallowable" / "not swallowable". This is reasonably because if they have started to swallow there is generally no way back because of the preys anatomy (Horns, Legs ..) and their jaws.
Since big snakes won't stalk and round their prey before attack they need to use first vision to do the above mentioned characterization. The vision of a standing human will be confusing and most likely distracting because the frontal view of an upright person of 1.8m would suggest a much bigger body in any other species that would be "un-swallowable" by the snake.
People that are attacked by big snakes with non-defensive intention were most likely not standing or walking upright. This fits well with the reporting of (little) children beeing caught and reports on hunting communities in south asia were more man than woman had been attacked by reticulated pythons ( http://blogs.thatpetplace.com/thatreptileblog/2012/01/30/people-as-python-prey-giant-snakes-attack-150-kill-6-in-philippines/#.VGMOtFaEW2x )
To lastly answer your question:
There have been reports on pythons and anacondas killing humans (most likely children) but they are rare and suggesting that humans aren't an usual prey of snakes. The rational above might clarify why this is the case.
