Because their are people who assume the world revolves around them and demand that everyone capitulate to their demands.
I don't want to discuss threads that involve XYZ, so nobody should be allowed to discuss it.
I don't like when people don't appreciate X brand of vehicle/trailer/motorhome and nobody should be allowed to disparage it.
I don't like it when people disagree with my favorite forum member and nobody should be allowed to disagree with them.
Remember that bratty little kid that would take is ball and go home if he didn't like the game that everyone else was playing... well that kid eventually grows up, joins a forum and complains about how someone else runs it.
Personally I find a couple threads that annoy me, so I just ignore them. To completely disagree with DutchmenSport, I absolutely cannot stand the petulant people who REFUSE to search for a thread before posting a question or comment for the hundredth time. When I see the same question posted all the time, I ignore them. I believe that a lot of people do that, therefore the question that was asked (and previously answered in multiple older threads) doesn't generate a lot of views or responses. But, rather than go on a thread and complain about how ignorant a poster comes across for asking the question rather than searching, I just ignore it. I don't demand it be closed... I ignore it.
That is the beauty of choices. You can chose to ignore some of the stupid people around... its pretty hard to ignore all of them, but you can ignore some of them.