Because it's a pro app with a ton of features and nearly unlimited capability to build effects, composite images, do motion graphics, and just about anything else. $1000 isn't very much for all those tools, but you don't need all those tools if you just want to make a home movie. You do need those tools if you want to approach the professional levels achieved by the major effects houses (most of which use After Effects)
At first I thought it was 199 because I was tired but it IS 999! Why does it have to cost so much?
Umm, because programmers have families, houses and cars, too? Because of that they need to get paid? Because it supports a ton of formats and technologies that involve third-party licensing fees? There's is a million more other good reasons, but your position is really not understandable. In terms of effects and compositing tools aimed at the professional market, AE is really at the lower end of the price range. It isn't meant to be a hobbyist tool, even though many hobbyists who appreciate its richness still are willing to pay the price and use it, too. If the price is too high for you, you can find enough tools from free video processing programs like Virtual Dub to tools bundled with disc burning programs (Roxio, Nero) to lower end commercial video editing programs (Premiere Elements, Magix, Ulead), but then of course you can't expect to get the same flexibility and support for file formats and you will often of course be stuck with canned effects or even have to buy additional plug-ins to create an effect that AE does in its standard package. After all, you get what you pay for...
As previously mentioned, $1000 is cheap for a professional-grade compositing program. You can get some machines
for doing stuff like this for hundreds of thousands of dollars.
Also AE lets you do an amazing amount of things and is an extremely powerful program. Considering what I can do with it, I am constantly surprised at how inexpensive it is.
You would want to use AE if you are doing professional-level compositing and motion graphics. But you're not; you're doing home movies.
You probably don't even need a program like this at all. You want a program that does video editing. Such as Premiere, or - since we're talking home movies - the other programs suggested by previous posters.