Because IS inevitably involves both optical and constructional compromises. Were we back in the film days or looking at cheap cameras/lenses then I have no problem with IS. When IS was introduced it was a life saver for photographers!
These days with higher end cameras and lenses I find that IS has more drawbacks than advantages. The impaired AF and tracking is more of a problem for me than learning to hold a lens steady or bumping ISO a little. So I do not like IS.
You probably consider me as mad as a box of Frogs ;D That's absolutely fine! However, if you do not live too far from South Wales I am more than happy to demonstrate the advantages of not using IS and, if we are lucky, introduce you to some other photographers who have benefited from disabling their stabilisers.
I have 4 Canon IS lenses (16-35 F4 L IS, 100-400 Mk2 L IS, 300 F2.8 L IS and 800 F5.6 L IS) - do I turn IS off to make a point or to spite myself? Err - no! I do it because I get better results - unfortunately turning IS off does not help with composition or inspiration - sorry! I have found a marked improvement, especially with moving or unpredictable subjects, so should I not share?