On PC with a dedicated graphics card, the increase in raw power required to go from 1080p to 1440p in Zwift is negligible. That’s why from GTX 960 upwards/newer there’s no point even bothering with 1080p. 4K requires a massive step up, which really isn’t worth the extra cost and demand.
Bear in mind that the profile (what you actually see in terms of detail and features) is unrelated to the resolution the game is rendered in. Apple TV gets Basic profile, which is the lowest tier. A GTX 960 gets Ultra profile, which is the highest. So there’s not really any comparison. If you want better graphics in Zwift, ditch the Apple TV.