"If it's too good to be true, it probably is.". Who has heard that before? Anyway, the only good types of traffic are from the main companies like Facebook, Google etc. - and even with those, there is massive danger if you don't know what you're doing!
Anyway, the lure of cheap traffic is simply the fact that website owners sometimes cannot afford the prices of major companies and/or they are just looking for something "way easier".
O.K., why might this traffic be trash? Well, some could very well be bots, not even human.
In another case, the source of the traffic is very "forced"(as in a pop-under). In other cases, as I suspect, the "forced stuff" could be fixed to where it seems like people really went there when maybe they were just trying to get away from the ad ASAP.