In the vastness of space, if supernovas and other explosions happen often, why can't we simply look up and see them?

Yes, explosions happen and emit tremendous amounts of energy but there are many reasons we can't see it.

Explanation:

Yes, in the vast immensity of space, novas, supernovas, and other such phenomena happen often.

So why can't we see most of them?

One reason is that the visible light spectrum within the total electromagnetic spectrum is extremely tiny. People can't see the vast majority of electromagnetic waves (we don't see the microwaves emitted by the microwave oven or the infrared in the convection oven, nor the x-rays at the dentist or the gamma waves that are emitted by the Sun). And the vast majority of the energy emitted from those explosions are not visible light waves.

Another reason is the distance involved. Think of this - an explosion will happen in a sphere (it'll explode in a 3-dimensional spherical way). As the energy moves through space, the amount of energy in each unit of space decreases exponentially. The equation for the surface of a sphere is $A = 4 \pi {r}^{2}$, so as the energy moves one unit towards us, the energy is $\frac{1}{4}$ as dense. Now take that over vast amount of distances and see that the energy will dissipate quickly.

Yet another reason is that there is a lot of stuff between Earth and where those explosions happen. Far from space being empty, it is filled with things that will block light, such as gas clouds, and also things that will bend light (both away and towards us) such as gravity fields of large masses (starts, galaxies, etc).