Others have pointed out the time and money drain, and while those are true, I don't have a TV for another reason.
More than any other single item that I'm aware of, a TV is a device to indoctrinate you into a certain way of thinking, and that thinking is consumerism. Everything you watch is an ad for something you don't actually need, and as time drags on, you'll slowly find yourself buying yet more stuff.
I find it extremely liberating to not have a TV and be able to think for myself about the things I do or don't want to buy, rather than have someone else tell me about it.
In my country, they are commonly referred to as "The Idiot Box". (And it's not because of the people you see on it...)