This is because HIV regularly changes their genetic code by mutation. A drug , commonly, combines to a protein having a specific structure but these mutations rapidly changes that very structures,therefore, now the drug does not bind to that protein of the virus and requires to develop a new drug. Same thing happens for antibody generation..... with new mutations every time body need to create new set of antibodies and withing that time span HIV binds to the immune cells. Moreover, generation of antibody for viruses is a bit complicated that bacteria or other pathogen with larger structure.
I am not entering in detail of the mechanism just trying to explain for general people.
I think, human race will never be abolished by any infection disease because the evolution, that's been happening every moment by accumulation of favorable minute genetic variations and mutations. There are a group of people who are resistant to HIV. Again comes the term "genetic mutation". This mutation caused the receptor of CD-4 ( immunity system) cells to change a manner that HIV can't bind to them and fails to make them AIDS patients.