Why does a particle-antiparticle collision produce $2$ photons instead of $1$?

In one of the questions that I did, it asked: What is produced when an electron and positron collides with each other? The answer is $2$ photons. Why doesn't it merge into one? After all, the photon with that amount of energy can decay back into a particle and antiparticle.

Is my understanding correct?

PS. I'm just a beginner learning about the standard model. Would be nice if the answer is simplified to a lower level. Thanks.

Answer

Photons have momentum $p=\frac{E}{c}$ (despite having no mass) Thus to conserve linear momentum, multiple photons are formed, moving in different directions. Also, photons are typically formed for low energy collisions. High energy collisions can result in exotic heavy particles forming. Furthermore, the annihilation (or decay) of an electron-positron pair into a single photon can occur in the presence of a third charged particle to which the excess momentum can be transferred by a virtual photon from the electron or positron.