Because we know how different things emit photons. We know a light bulb emits photons in all directions because we can move around and measure it. And we can see the photons being emitted from objects receiving the initial light bulb's light as well so we know it's emitting light in that direction as well.

The idea that photons are only emitted if they hit something also doesn't make sense because of power usage and how we know particle physics work.