Simple, because its slow, and depending on your source, can cause issues sometimes streaming (denoise when used with crf can drop bitrates alot on a noisy source, like an old grainy movie which can cause crf to bloat bitrates and file sizes quite a bit) . Its the same reason we chose single pass with the appleTV preset vs. 2 pass abr, tradeoff quality for time. Its a good tradeoff for 95% of the people using it. They will never see the difference or if they do, its not worth the extra encoding time.
The Built In HandBrake Preset is a click and encode solution. Exceptional quality at a decent encoding speed and reasonable file sizes. It streams well in almost all circumstances and more importantly is predictable with all types of sources. Plus it uses CAVLC which fits the AppleTV's published specifications. The settings I outline above use CABAC which is not officially endorsed by Apple for the AppleTV. So we do not ship a built in preset with CABAC. However believe me when I tell you that when properly setup, CABAC encodes work just fine on the AppleTV. I have done and watched hundreds of them since the ATV was introduced.
Always remember that the Built In Handbrake presets targeted at devices are a compromise. Often times taking storage space of the targeted device into account (hence the low bitrate on the iPhone preset which has dismal storage capacity) , encoding speed, and quality tradeoffs. Ultimately there is no "correct" setting. Each person has to determine whats most important to them. The presets are just a general starting point. Most people never go past the presets. But if you do, the options are almost limitless. The flip side to that is you can also totally screw up device compatibility pretty easily.
