I've been studying this phenomenon for years. Your observations have been validated by numerous sources. The best scientific information available today proposes that the noise from their engines and exhausts has resulted in significant hearing loss. The highly polished chrome, blinding as it is, has similarly resulted in retinal damage and visual loss. The vibration of their vehicles has reulted in carpal tunnel syndrome and other nerve damage. The sum total of this has lead to sensory deprivation, altered perception, and global sensory numbness akin to prisoners in solitary confinement for long periods of time. It is, in essence, "shut in syndrome".
Harley riders therefore, likely do not see the wave. If they see it they do not understand it. If they understand it, they do not recall how to respond to it. Simply put, it's just not their fault. They are victims of vehicular retardation. I say: keep waving my friend. It's the right thing to do. One day there'll be a cure...like trading in the Harley for an FZ6.