Is there a cure for depression?
Do you mean currently or potentially?
This is my understanding of the difficulty. Our understanding of the human brain has increased massively in recent years, especially thanks to the expanding brain meme science, but we're still relatively dumb regarding it. We know a lot about things we can trace to physical manifestations, like when you raise your arm or whatever, but to understand the complexities of what drives human emotion and sense of being we're chasing symptoms, and hoping that we can identify individual causes or at least enough of ones that we can manage the problem on a case-by-case basis that's not inherently repeatable.
Our paradigms that examine mental illness still approach it from the disease or disorder view in which like the rest of our medical science we can then find and test solutions for. But we don't actually understand the mind part of the brain, let alone the self part of the mind, we have many theories and many of them can help, but it's not something can visualize and examine under a microscope.
Heart attacks were once far more fatal than they are today, while lots of technology has improved that situation, one hurdle was a fundamental misunderstanding of the heart. Even when many of the necessary surgical abilities were already available. Except it was thought that after such a potentially fatal event you needed perhaps a month or two of bed rest, and a slow soft recovery. Now it's seen more as a literal "attack" and once you're past that, and any necessary surgery recovery, you should return to normal behavior as easily as possible. They've been finding similar things with certain cancers. Bed rest is often a good thing, especially in the immediate aftermath, but too prolonged of it stresses your system worse and hinders recovery. Which often lead to people in the olden times having almost immediate events again, and the theory being that they exerted themselves too quickly, not that they had atrophied themselves for too long.
Even with the means it took us a while to learn these from improved abilities to study the systems. And the brain is, for our current perspectives, infinitely more complex than most of our internal systems.
Curing "depression" is like curing "cancer" in that it treats all forms equally, when they aren't. And we have the benefit of knowing the essential cause of all cancers. Mental health issues are often guesses based on seemingly sound theories but ones we can't exactly "look at" like we can most everything else.
Unless you're Phineas Gage or something.