Most people don't die from HIV now so it's not the death sentence it used to be as the medications we have are good for treating HIV. But yes medication isn't a cure but we're getting closer to one. Biochemistry is very complex and we rely on studies to enhance our knowledge, however not all studies are high quality. Biochemistry is so vastly complex that it's slow going. For example cosmetic conditions such as male pattern baldness are still not fully understood which is why we don't have any drugs yet that were designed to treat baldness, only repurposed drugs such as Minoxidil and Finasteride.

I have these charts from Roche, which are just a few of the main biochemical pathways in the human body, so whenever I get an idea that solving a biochemical problem is easy I just look at the charts >

http://biochemical-pathways.com/#/map/1

There's many 100s or thousands more pathways and more to discover. Nature has millions of years head start on us.

This is a good book to have a look through just to get an idea of how mind bogglingly complex biochemistry is

Biochemical Pathways – An Atlas of Biochemistry and Molecular Biology 2e by Gerhard Michael

I've used that book to look up some biochemical pathway I was interested in and it's not even in the book. Plus depending on brain region or area in the body the biochemical pathways don't work the same way. It's beyond my comprehension anyway, I'm just speaking from a perspective of doing this as a hobby so I'm sure some experts can come in here and explain further if they wish.

The medical breakthroughs reported in the general media often cite poor studies, interpret the studies incorrectly or hugely over simplify biochemical processes. So we're led to believe Serotonin is the happy neurotransmitter and Dopamine is the reward neurotransmitter and it's all just that simple when it's anything of the sort.

Biochemistry is fascinating but the reason some conditions are incurable so far or we wonder why we can't just cure baldness, it is because this is vastly complex and requires high quality methodological study. Then meta analysis of the studies if there are enough.