Animation of immune cell attacking invader, used as analogy.
Full comment
This is because HIV is a brilliantly designed virus (of course it evolved, it's not designed. I don't know a better way to say this though). It's a third-generation retrovirus, meaning it defeats internal cell safeguards in a very clever way, meaning that if one cell of a few types comes into contact with ~10 virus particles, you effectively have AIDS. For the flu we're talking hundreds of cells needing to be in contact with thousands of virus particles. It evades being a target for vaccination by using internal body networks to transmit itself as opposed to the blood. The immune system inside our bodies has a sort of chemical protocol for one cell to exchange data with another. One cell approaches another, presents a specific molecule, and the cells link, exchange something (we don't know what, or why, or how for that matter), and the HIV virus slips along. This means HIV looks at the immune system from behind all the detection mechanisms. You can look at immune cells as a mine : they have lots of traps on the outside. Trigger one of them and a massive army of huge, very angry, and very dangerous cells will overwhelm whatever triggered it ( this video ), and a large part of things nearby that didn't trigger it too, usually. So where does HIV hide ? Simple : on the inside of the mine, transmitting itself to other mines when through the mechanic that comes by to repair the mine and doublecheck things every now and then. There's other virii doing this, however they don't generally