pair production - Why during annihilation of an electron and positron 2 gamma rays are produced instead of 1?

$1 \gamma \rightarrow 1 e^- + 1 e^+$ (pair production)

Then why

$1 e^- + 1 e^+ \rightarrow 2 \gamma$ (annihilation of matter)

instead of

$1 e^- + 1 e^+ -> 1 \gamma$ ?