Psych Grad Student here(well u kno me) but also as a fellow mentally ill and depressed person I think I can give a bit of a mixed set of answers to your questions.
It's just that I don't think people know what truly causes mental illness
Science does and it doesn't. The patterns are there for many disorders, but there is no obvious concrete cause we can point to. For instance, with a disorder like DID(dissociative identity disorder) the vast majority of people with it have experienced childhood trauma, so there is a clear casual link. Moreover, for some really rare cases of OCD, it can actually be a brain lesion, which can even be surgically removed to treat the OCD. But for the most part we have no idea what causes what exactly, since most disorders are simply too multifaceted. For example, MDD/PDD(Depression) have numerous genetic and environmental facets which cause the disorder, and that can range from body health, to brain inflammation, or gut inflammation, and so forth. Furthermore, it can also be a result of abuse, heartbreak, grief, and more. As of the current moment, the science is just not able to pinpoint an exact cause.
What is therapy supposed to do?
Several things, and it varies per what form of therapy and the general practices of the clinician. DBT, CBT, and ACT are some of the more common types of therapy, each serving differing functions for differing disorders. General psychotherapy(talk therapy) can also vary based on the beliefs of the practitioner. For instance a humanist inspired by Roger's is going to enjoy repeating back in questions what you say, whereas a behaviorist inspired by Skinner will focus more on your surrounding conditions and how they might influence you. Which is the best one? Well none of them, it's really to each their own. (This is cap biospychosocial model on top lmao).
This is even more complex, because meds differ greatly from one another, but often can feel identical if they don't work for you. It also matters whether they are first line, second line, or third line medication, as each time you go up the meds progressively become stronger and higher risk. SSRIs will obviously differ from NDRIs, antipsychotics will differ from sedatives, but the whole thing becomes a jumble since sometimes you will receive multiple meds(assuming they interact safely with one another) especially if you have comorbid disorders. This really requires you to communicate with your doctor or psychiatrist and to do your own research regarding the specific medication you are taking.
No one can explain how therapy even works, unlike other verifiable, objective health treatments. How am I supposed to trust the process if we don't even know why it works? Sounds like a waste of time and money to me.
Can we explain it? Kinda. But therapy statistically does work(though I have problems with most of the reporting since I think it's heavily skewed if you don't account for baseline moods). Does that mean it will work for you if you have a more serious disorder or trauma, probably not, but for a broad population we can at least measure success by just interviewing people and monitoring how they improve on diagnostic tools. Unfortunately, therapy isn't a guarantee, and unlike fixing a bone, the brain is just too complex to answer this simply. As for whether it's a waste of time or money, that is entirely dependent on you. And that's not me saying "be more positive", it's just me pointing out that if you don't think therapy is going to work, then it's probably not going to do that much for you.
I want to get better. I just don't think therapy works, and I don't understand why people think that means I'm against getting better. I think it's more likely there's just no help for the mentally ill and no one likes to admit it.
I also personally don't think therapy works, but that's just because I'm a full blown nihilist who is uncooperative and resistant to treatment. I've been in therapy for most my life(since age 6) and it really hasn't done shit for me. Personally, I find that therapy and medication are often excuses that loved ones give for not knowing how to actually support us emotionally. It's easier to default responsibility to a professional, than it is to actually be the emotional support and connection someone needs.
Anyways hope this kinda helps. Or maybe it doesn't lol. The answer for everything is sadly "brain too complex we just aren't sure, even if we have ideas as to why".
Also if you ever want to talk more specifically about this you have my disc.