There are good therapists (and doctors, and scientists...). And the vast majority are not fraudsters.
But all psychics are bad at their job, in that they are incapable of doing what they claim to do. Every one of them is a fraudster.
I read that depression is what the mind/body does when it needs to step back and re-assess the (life) situation, because whatever it's been doing so far obviously isn't working. I wish I could link to it.
However it's important not to generalise on this idea too much. Depression is a mental illness and that means it comes in great variations, including things that are just unique or go past theory. More importantly, it is very possible that the above described process is just a secondary or tertiary factor in the grand picture.
All models are fundamentally wrong. That is what is meant by the phrase "the map is not the territory", it is only a matter of degree and scope. Building a better model of reality is a difficult and painful process which manifests as depression [0]. In many cases, it is not worth the added suffering to force someone through this process just to understand what you're telling them.
> The psychic preys upon vulnerable people seeking advice about problems in their lives
One could say the same for therapists. There are quite a lot of bad therapists, trust me. Now, I'm not saying that psychics or astrologers are comparable to therapists in terms of overall efficacy, but if people are inclined to believe in that sort of thing anyway and the advice they get helps them, then fine.
Yes, it is a problem when fraudsters take advantage of people, but that goes for literally any practice including therapists, doctors, and scientists.
[0] This is more of a pet theory based on experience than anything I can back up through study.