Granted overtraining is a thing but undertraining or no training is also a thing. At the level most people are at (sedentary or undertrained) coupled with the fact that people have reasonable intuitions as to when they feel tired or not, the risks of overtraining is signficantly less that the benefits of regular exercise both physically and mentally.

You do mention inflammation in a previous comment and overtraining increases it but does the negative consequence of inflammation due to overtraining on how one feels subjectively outweigh the cascade of endorphins and neurotransmitters in response to exercise that correspond to an uplift in mood or an alleviation of stress? I'm not sure but in the given context (original post) your point is valid but of much less concern probabilistically and physiologically.

I do grant however that that is only my opinion and very much up for debate.