It just isn't as easy as it looks.

In fact, things that are a great deal more complicated can also be done, such as really fantastic real-time echo cancellation or feedback prevention in a large room with many speakers and many microphones. It just isn't as easy as you would naively think; everything that just leaps to mind (especially if you aren't particularly knowledgeable about signal processing) don't work. You'll pay big bucks for the system that can do that, and there's a reason for that.