Because like most viruses it mutates. Which is why we don't have a cure for other viruses like the common cold or the flu. We can come up for a vaccine one year and then it changes the next.
Why isn't there a cure for AIDS?
💰 and GREED
I think medical/doctors/drug conpanies has a cure for 85% of all diseases, especially most cancers, but 💰and GREED keeps them from giving the cures to the world.
Either they still haven't found a real cure or like cancer also, they may have a cure and won't produce it because they make too much money with the current treatments that rarely work. You would think that with all the scientific power out there that they have produced a cure but the almighty dollar keeps them from producing it. This article is pretty interesting to read though.
https://en.wikipedia.org/wiki/HIV#Genetic_variability
This is a retro virus of extreme mutability and capable of being a reservoir virus in the immune cells. This is an RNA virus that uses reverse transcription to hide in the cellular DNA. The viruses genetic material, a single strand of RNA, enters the nucleus of the target cell and with it's own reverse transcriptase enzyme runs off a copy of cellular DNA and the another copy of DNA is made from that first cDNA copy. Then this double copy is inserted into the cellular DNA where it can emerge later to replicate the virus. The reverse transcription is extremely error prone which means that medications can be useless against the virus, as it changes just by the process of reverse transcription. This is the lysoginistic virus cycle and makes treatment of such viral infections difficult. If HIV did not target immune cells then the body would have it's defenses up for the opportunistic infections that occur during AIDS progression.
There are other retroviruses that we have no cure for but they usually go through the lytic cycle and do not attack the immune cells. Remeber that the next time you get the flu.
There is more money to be made in all the medicines that are for cancer, AIDS, etc., than the cure. So, if you had a steady income from the scripts, you would want to keep that momentum going, rather than have a cure available so the income would stop once they're cured.