A fully automatic ETTR mode is hard to define because ETTR is not a single objective target. The key problem is deciding which highlights are acceptable to clip and which are important to preserve. That is a creative and scene-dependent judgment, not a purely technical one.

There’s also no universal agreement on what counts as usable photographic dynamic range. Different photographers have different tolerance for clipped highlights, shadow noise, and artifacts depending on the subject and intended output.

So while a camera can meter quickly, it cannot reliably know your intent: whether a bright reflection, light source, or specular highlight may be safely sacrificed, or whether every highlight must be retained. Because ETTR depends on those choices, automating it would often produce results some users consider wrong.

In short, the limitation is less about camera speed and more about ETTR being subjective rather than universally measurable.