If, just as a thought experiment, it was possible to treat a disease quickly with a cheap drug or intervention, for example a cheap herbal drug that is available patent-free and for mere cents per liter, there would be no business in it, so no company could viably pursue this. The market selects for treatments that give a lot of profit. That always or even merely "usually" automatically somehow translates into the best option from a public health point of view only if you close your eyes and make lots of assumptions about how great the market magic works - in a market with extremely unequal information and very high pressure on customers (when you are sick).
On the other hand you are right that symptom treatment usually is the best we can do. The body is built during the early years and from then on it's maintenance, also what the body itself does and not just the doctors (it does not regenerate most structures, only repair). Just more and more ever more horrible hacks, workarounds and kludges and patches delaying the long decline more or less well. Or as most drugs do, don't even repair anything, just push levers and buttons in the body, but it doesn't do anything structural unless the body does it on its own. So if the structures don't regenerate the button-pushing drug has to be take lifelong.
That is so fundamental to complex life forms that current medicine simply cannot work against it. Maybe never, because replacing a structure in the grown body is much more difficult than growing it with everything else. For example, the difficulty of finding the path for new axons after a spinal injury is much greater than building everything together during very early development. Medicine would not just have to achieve what nature already does (when the body is assembled) - it would have to become far better! Repairing something often is much more difficult than starting from scratch.