Welcome to the forums!
I would agree with Rick’s observation that you should not expect a response from a “Canon rep” in these forums. I would suggest that you contact them in writing. Yours is a highly technical question pertaining to design decisions, so I would not expect any company to reveal proprietary information regarding intellectual properties.
We, as users, can only speculate on the reasons. I do not think Image Stabilization is really needed in wide aperture lenses shorter than 100mm, because your shutter speeds would typically be faster than 1/100 of a second.
Under some shooting conditions and scenarios, having IS enabled could create undesirable effects such as soft images or even pulsing. One frequent scenario that I’ve heard people complaint about with the R5 when shooting video is how the image shifts. When they post examples of the issue, they are focusing on fairly distant objects, like hundreds of yards away, if not miles away. They are most often shooting video handheld, which is why they feel that they should have IS enabled.
This brings me to my next point. The primary job of IS is not to minimize motion blur in your photos. The primary job of IS is to provide a stable image in the EVF/OVF for the photographer, but more importantly for the AF system.
It only takes a small fraction of a second to capture most images. This means that the IS system probably spends more than 99.99% stabilizing the image for the AF system. If your shooting conditions warrant the need for IS because of slow shutter speeds, then you probably need a tripod and should turn off the IS, anyway.
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