Agreed, but part of the problem is that they're often framed by their proponents as a miracle drug that can cure you of depression, anxiety, and fear of death. So people with those problems will often self medicate with them.
And for some people, it totally works. But it can backfire tremendously, and actually worsen those problems. There's a psychonaut meme that there's "no such thing as a bad trip, just a difficult one", but that's absolutely untrue.
I think psychedelics have incredible therapeutic potential, though, and I'm looking forward to a world where they are legal, well tested and researched, and usable in safe environments with trained and compassionate professionals.
Once you have some sense of what to expect you can more readily use your judgment as to whether you're in the right headspace. I find it can be quite helpful to mitigate mild/moderate anxiety or depression.
But therein lies the crux of this kind of reasoning: those kinds of requirements are vague and highly subjective - what does "mentally balanced" entail? who determines that?
It would require a high level of self-awareness and even then, some woundings/mental illnesses may not be apparent until some sort of traumatic event (such as a bad trip). It's incredibly difficult to know ahead of time what kind of reaction someone is going to have to such powerful mind-altering substances and IMO although the advice of "set and setting" is important, it in no way will prevent all possible issues from happening.
I know its easy to say in hindsight, many of us tried those things when we didn't think about it, and it worked better for some and less for others.
I think the old set & setting has some proper meaning here and can vastly bootstrap the incoming experience. Having a calm, even soothing environment, no negative outside influences like badly tripping / drug rejecting friends who can drag anybody down to mud. If one is really unsure, have some friend 'spotting' for you and drag you back and calm down if bad trip comes. If one is in bad mental place, just don't do it, it ain't alcohol which can be taken almost on any mood.
Personally, I've never had a profound trip unless alone, relaxing on the bed with nice music in the background, but that worked for my specific personality.