Why does HAART combine several anti-HIV drugs instead of relying on one?

1. Used on its own, any single anti-HIV drug meets a virus with a high mutation rate. 2. The virus then develops resistance easily and rapidly, and the drug stops working. 3. HAART, highly active anti-retroviral therapy, instead gives a mixture of different drugs together. 4. Because the mixture attacks different stages of the replicative cycle, resistance to all of them at once is much harder to acquire. 5. Solubility and seasonal timing play no part in this reasoning, so those options are invented. _Source: OpenStax Biology (1st ed., CC BY 4.0), Ch 21 "Viruses", section 21.3 Prevention and Treatment of Viral Infections_