The best answer was already given:
Technology changes due to competition. If you look at the market it is always a game of oneupmanship. Company X comes out with 120hz TVs, then company Y comes out with 240hz TVs. We see declining sales at theaters, then a big uptick in 3D at theaters, and then find a way to bring that technology home. This opens up a whole new line of sales of glasses, new players, new TVs, etc.
You buy an entire collection on VHS, then replace it with DVD, only to replace it again with Blu-ray.
Yet, your car gets driven and must be replaced and is constantly getting upgraded, as are most of your appliances and other things you own.
Unlike your PC, you should be able to use and enjoy the TV you have forever. Just because something new comes along doesn't mean you HAVE to upgrade... But, you sure as heck will be marketed to like the upgrade is a requirement.
I would say, with PCs and software requirements, the upgrade is forced upon users far more often that with other technologies.