[apitaru]

By making the collision predictable [edit: interestingly, larger than I would have imagined needed], the gamer knows exactly why he/she failed. There's 'honesty' in the directness that most games lack (and therefore gamers crave).

And then - by allowing a quick restart, any frustration is overshadowed with the desire to try again.

There are other interesting fine-details. For example, notice when the bird starts to dive (an experience game developer pointed it out to me). It happens when the bird goes under the last flap-point. This allows for a much deeper control system than the one-switch interaction suggests. This depth is intuitively grasped by the gamer on some level, providing more fuel to try again.