Why is a retrovirus harder to treat than other viruses?

They can hide in DNA: Retroviruses have the ability to insert themselves into the dna, effectively "hiding" themselves from the immune system. Also, they rapidly mutate which can prevent the immune system from attacking them.

Wrong premise: All viruses are hard to treat. In fact very few viruses have any treatment. It is not accurate to say that HIV is harder to treat than other viruses. It is hard to treat because it is a virus and viruses mutate and become resistant to antivirals.