By a handshake we acknowledge our equality with others. We reveal our humanity. We convey how strong we are, how nervous, how aggressive or passive. There is bold physicality to it. For these and other reasons, Popes never shake hands.
Kings never shake hands.
Even mothers don't shake hands with their own children. Namaste is cosmically different. We all namaste before the Almighty, a holy man or even a holy place. The namaste gesture bespeaks our inner valuing of the sacredness of all. It betokens our intuition that all souls are divine, in their essence.