Why is this surprising? "Use it or lose it" may be a cliche, but it's true; if you don't keep some faculty conditioned, it gets "rusty". That's the general principle, so it would be surprising if this were an exception.

The age of social media and constant distraction already atrophies the ability to maintain sustained focus. Who reads a book these days, never mind a thick book requiring struggle to master? That requires immersion, sustained engagement, persevering through discomfort, and denying yourself indulgence in all sorts of temptations and enticements to get a cheap fix. It requires postponed gratification, or a gratification that is more subtle and measured and piecemeal rather than some sharp spike. We become conditioned in Pavlovian fashion, more habituated to such behavior, the more we engage in such behavior.

The reliance on AI for writing is partly rooted in the failure to recognize that writing is a form of engagement with the material. Clear writing is a way of developing knowledge and understanding. It helps uncover what you understand and what you don't. If you can't explain something, you don't know it well enough to have clear ideas about it. What good does an AI do you - you as a knowing subject - if it does the "writing" for you? You, personally, don't become wiser or better. You don't become fit by watching others exercise.

This isn't to say AI has no purpose, but our attitude toward technology is often irresponsible. We think that if we have the power to do something, we are missing out by not using it. This is boneheaded. The ultimate measure is whether the technology is good for you in some particular use case. Sometimes, we make prudential allowances for practical reasons. There can be a place for AI to "write" for us, but there are plenty of cases where it is simply senseless to use. You need to be prudent, or you end up abusing the technology.