I suspect the answer to "why do they do it?" is because they can and they want to. If someone wants to trace the ancestry of people they are not related to, for their own amusement and to pass the time, why shouldn't they? If someone wants to try and connect 300,000 people together, again for their own amusement, why shouldn't they? No one is forcing other people to look at their trees - if you are clicking on other people's trees then you have chosen to do so.
As to the inaccurate trees on Ancestry - I wonder whether the owner of many of these trees started out believing Ancestry's advertising hype and thinking that the "hints" were Ancestry telling them who their ancestors were. By the time they realised the error of their ways they had built a mess of an inaccurate tree. On realising how much effort would be needed to correct it they lost the will to live (and who can blame them). So they either stopped, or stopped that particular tree and started another one. An inaccurate tree on Ancestry therefore does not necessarily mean the owner is a complete idiot.
As to similar threads on here - if you clicked on the thread to read it, you chose to do so. If you are reading every single thread that is posted here then you obviously have a lot more spare time than I have!
If we all thought exactly the same things all the time, what a terribly boring place the world would be.
