In cameras like the EOS M6, the “5-axis” stabilization offered for video is typically digital/electronic stabilization, not in-body sensor-shift stabilization.

That matters because digital stabilization works by adjusting each frame—shifting or rotating the recorded image based on motion data and/or image analysis. This is useful when you have a sequence of frames to smooth out, which is exactly what video provides.

For a single still photo, that kind of frame-to-frame correction doesn’t really apply. Instead, stills benefit more from optical stabilization in the lens, or from true in-body sensor-shift stabilization on cameras that have it. Those optical/mechanical methods generally preserve image quality better, which is especially important for still photography.

So the reason is less about raw processing power and more about the type of stabilization being used: video-only 5-axis modes are often electronic stabilization designed for continuous footage, while still photos rely on lens-based or sensor-shift stabilization for better image quality.