Because in quantum mechanics, the behaviour of particles is governed by probabilities. Not just probabilities like a die roll, which we can in principle predict in a perfect simulation, but really random, with no hidden information at all.

If you have a point source of light (or gamma rays), for example a black body, it will radiate equally in all directions. On a large scale this seems familiar. But thanks to quantum mechanics we know that radiation has to come in discrete packets called photons.

A point source of gamma rays will appear to radiate out in all directions, but if you reduce the rate of emission you will be able to pick out the individual photons. If you slow it down enough you will see photons emitted one at a time.

A single photon cannot radiate out equally in all directions, it has to be in one place when it is detected. So you will measure individual photons coming off at any angle, but distributed in such a way that if you measure a large amount, they will form a nice, relatively uniform pattern.

There is no reason electron-positron annihilation should be any different in this respect than black body radiation. The angle of emission is due to chance, but conforms to a definite probability distribution, and if you measure many of these emissions you can predict the distribution of the bulk of the photons.