One reason is the same as authors who don’t want their books trained and actors who don’t want their likeness trained. If that content is valuable it allows google to realize that value with no return to the creator.
The naivety is assuming a small number of people making access hard will affect the value they're able to realize.
I bet the reasons vary from person to person. My reason is because I think that these AI systems pose too great of a risk to society, and I want to make sure that I'm not helping them in any way.