I can't say I'm surprised by this. The brain is, figuratively speaking, a muscle. Learning through successes and (especially) failures is hard work, though not without benefit, in that the trials and exercises your brain works through exercises the 'muscle', making it stronger.

Using LLMs to do replace the effort we would've otherwise endured to complete a task short-circuits that exercising function, and I would suggest is potentially addictive because it's a near-instant reward for little work.

It would be interesting to see a longitudinal study on the affect of LLMs, collective attention spans, and academic scores where testing is conducted on pen and paper.