Why does HIV have so few spike proteins on its surface, unlike any other virus?

The surface of a virus is covered with spike proteins that are used to penetrate and infect a host cell. Spikes are also targets of the immune system as antibodies can bind to them and neutralize the virus. Most viruses have very high density of spikes of their surface. However, the HIV virus has spike density almost two orders of magnitude smaller than other viruses. This unique feature of HIV has defied explanation since it was first discovered.
Using approaches from population dynamics and non-equilibrium statistical mechanics, we show that spike density will determine the affinity of the antibodies created against them. This imposes evolutionary constraints that led most viruses to develop very high spike density. We suggest that HIV, because of its unique impact on the immune system, evolved to be a low spike density virus.