i assume you are asking why watching TV makes you dumber, because you could be doing something constructive with that time (basically anything that involves active problem-solving) instead of sitting on your ass staring at a screen while being mentally hand-held throughout (Jersey Shore). it is a fantasy scenario that the director and editors create to keep your interested, and not a reflection or indicator of real life situations, so it's not good from an experience or learning standpoint, basically the only arguments one could make
BTW I didn't mention movies per se for a reason, they technically count as TV at home but a well-made movie is generally much more intellectually involving for the viewer than ANY tv show, which is made with leading into commercials and ads in mind, and keeping you hooked with any BS tactic they can pull as long as possible to watch more commercials and ads.
I'm not saying TV isn't entertaining, but that's the problem, it's all bang and no buck at the end of the day when one's vegetated in front of the screen for hours
there are some exceptions, especially in small children's TV, but once you get to 9-10 year old cartoons, it's all bull even there.