A TV now is more like a large monitor to hook other things upto, whereas in the past the TV set itself was responsible for decoding the broadcast stream etc.

A TV still has it's advantages however, they are cheap for their size because the resolution can remain relatively low (imagine how much a 42" retina display would cost).

There's also a big social aspect that a laptop does not address, you wouldn't want to bring your friends round for a movie or a friendly game of wii sports and all crowd around a laptop would you?