with aTV, developers can re-use most of their code for iOS apps, in other words most of the code is good for at least 1 billion active devices.
with smart tv, they have to develop an app for each brand, which might only be a few million units,
if you had to focus development on 1 platform with 1 billion users, or 5 platforms with 200 million between them, which would you choose to focus on?
Apple also has an established and reliable way to make money from your apps.
although as said before
apple doesn't support the video codec that YouTube uses on any device, so you're going to get better looking video directly on your TV.
and amazon prime, I have to search for better versions occasionally, I've had a couple times, that the aTV will have 4k, but not HDR. While the same show on my TV is 4k HDR.
Not to mention having 2 versions of every show, and it seems they don't want you to find the 4k version, I'll have to search "show name uhd" because "show name" only shows the HD versions. but that's on both my aTV and the TV.
as opposed to Netflix which will just stream the best version your device and network can play.