Medicine got its prestige by being able to diagnose definite problems with definite tests and offer definite treatments with definite results
"definite" tests are extraordinarily the rarity across all medicine. the rest of the body combined is more complex than the brain, and you often need to look at how the rest of the body interacts when you're solving a problem, and sometimes you even need to factor in the brain because stress can cause physical symptoms, everything interacts and you can't just separate one aspect and disproportionately scream at it.
it's simply that we've been studying the rest of the body for a bit of a longer time than we have mental stuff. in the '60s, aspirin was in a similar place as SSRIs are right now.
my conclusion is that science is never a clear thing because it is science. we document what we get in and then what we get out of it. we can experiment to show there's a result, and that doesn't mean we have anywhere close to a complete model of it. there's always pretty uncertain areas to explore, and it's often just better than nothing.