I think an argument could be made about the unneeded yearly updates of the iPhone, but a huge difference between the iPhone and the ATV is competition.You kidding right? That’s like saying why does the iPhone need to be released every year.
Yearly updates from Apple and its competitors push higher and better specs, newer features, and new designs.
That just isn't the case for streaming boxes.
Besides, the A10X still outperformance the competition, and in some cases, it isn't remotely close. Features wise, there really isn't too much HW features being offered that can be upgraded compare to phones and other mobile devices.
The one thing (only thing?) that the current generation of ATV is lacking is the newer HDMI standard, a relatively minor HW feature compared to stuff like cameras, antennas, charging features, etc. of a phone.