Because your TV is either (1) a basic model that didn't come with an S-video input or (2) it predates the introduction of the input, which first became available about 1986-87.
It doesn't have to be a high-end TV to have s-video. I have a 12-year-old bottom-of-the-line 20-inch Panasonic TV that has s-video. It's entirely dependent on the model, but I would say that these days, only the absolute entry-level TV models are without s-video.