The biggest reason is that there's value in owning the user experience. Before streaming video took over, cable providers tried to put TiVo out of business by offering their own (crappier) DVR products and gating TiVo and DIY products like MythTV behind a flaky, poorly-supported product called CableCARD.

If you own the user experience, you can push users into _your_ most profitable offerings. And it works shockingly well.