Because you are making it a blanket statement when it isn't one.
Players develop best when they face the appropriate level of competition. As the more knowledgeable posters explained in different Slafkovski threads where they were silenced by the popular posters collecting likes making baseless claims, this is backed by years of studies in athlete development and the benefits of meaningful competition in youth performance.
For some players, the NHL will be appropriate, for others it won't. Slafkovsky was clearly in over his head at the time. I guess to a majority of posters, it was sexier to imagine this guy was going to become the next Jagr roaming aimlessly on NHL ice.
Now, his development has been stunted because he was rushed. A Jeff Gorton well-known specialty.