Why do Photons Exhibit Quantum Behavior?

Please explain this then, other than an ad hominum.

Explain what?  Quantum Theory?  In short, Quantum means the smallest amount of something.  So, any theory about the smallest amount of something is a Quantum Theory.  I hope today's science lesson taught you something. 

Quantum mechanics are at the basis of many semiconductive effects, and so, are the basis of current electronics.
Photons may be massless, but its not void of momentum. The equation E^2 = (m₀ * c^2) + (p^2 * c^2) , which is the actual general equation for energy in relativity (from which it can be simplified in many cases to the E=mc^2 equation) gives us a little hint at why. That is how light sails can propel themselves.

I am aware of how Solar sails work; I referenced them in the original post with an (admittedly vague) explanation.
Quantum mechanics are offered as an explanation of how electronics work, but the mechanics themselves are not necessary knowledge. Even so, it would not follow the mechanics we use as a model are complete or fully accurate: Newton is all the evidence you need of that.

Computing is impressive, though I'm not sure how relevant quantum theory is. Quantum computers are still strictly theoretical.

Actually there have been a few quantum computers built, the probelem is that for them to do anything useful programming needs to be reinvented.

In any case, quantum theory was used to invent transistors which are the components that make computers possible.