Because Apple's marketing goal is to simplify, not add more. In addition they want to draw you into the Apple brand. They want you to have the, "Apple products

Because Apple's marketing goal is to simplify, not add more. In addition they want to draw you into the Apple brand. They want you to have the, "Apple products work great with other Apple products, and I already have all these Apple products, so I might as well choose an Apple product when I need something to do X" mentality. They want full hardware control. A small box wont do that. Another small box will have millions of combinations of other hardware attached to it that may behave in unexpected ways. If they sell you a TV instead, then they can ensure that they're the central hub (and likewise, that everything "just works").