@FocusSwordsman Why do most complex organisms only have two eyes, and only in the front?
That is a very good question. I'm going to avoid taking the microbiologist answer of "because multicellular animals are weird" and try to give some good concepts. We'll break this down into a few different questions and I'll try to explain the ideas behind each.
Why eyes to begin with?
Eyes are really important for most organisms, but almost none so much as humans. Our eyes give not only light input, but also allow us to interact socially and detail written information and so such humanities nonsense. We think of them as all important, whereas a dog might sooner go blind that lose its sense of smell.
Why do we use light for one our senses? Above water, it travels farther and faster than sound, pressure, heat, and molecular diffusion (smell/taste) and it allows us to perceive minute chemical differences in materials without ingesting them. (There are lots of chemicals that have colors outside of our range of vision, then tend to show up white or clear depending on their state.) This means that we have access to information more quickly and at a greater distance than any of our other senses.
Why in the front though?
Most animals have optic sensors near their brains to allow for optimal reaction speed. There is a maximum speed that your nerves can transmit information at, and in order stay relevant to something that is moving at the speed of light, they have to be as close to the brain as possible.
Human nerves can transmit information from 30-120 meters per second. [Bullock, H, (1965), Structure and Function in the Nervous Systems of Invertebrates], compared to light's blisteringly fast 299,792,458 meters per second [google.com].
This means that any eyes that aren't optimally close to the brain are going to suffer from some serious lag in responsiveness. Well, more so than we suffer from already. Brains, for whatever reason, are usually situated in the front of the organism.
Why multiple eyes?
Ok, so being able to see and interpret light is a good thing. So why in the world do we have more than one? It's not like we are going to be able to react soon enough to quick light stimuli with our signals to our muscles creeping along at a mere 120 m/s (which is a cool 268.432 mph for our imperial friends) so why do we need the same input multiple times? Well, it's not the same input exactly.
You've heard this before, as humans have excellent depth perception being hunters, but the slight difference in the image produced by two eyes that close to each other allows our fantastically bored brains to infer the difference in distance of multiple objects. This is primarily hunters though, as most herbivores have shifted their eyes to the sides of their heads to create a much larger field of vision but at the price of their depth perception.
The reason for multiple eyes is generally to create better depth perception or to increase range of vision. Outside of that there is little reason to generate new eyes, (though the infrared sensors in snakes could technically be considered eyes as they interpret a specific set of low wavelength radiation. I'm not a herpetologist though.)
Ok, but why two?
Two eyes? Oh, right. That thing that happens in most mammals, reptiles, amphibians, fish, birds, and almost everything with a vertebra. The first things with vertebras probably developed two eyes (because of bilateral symmetry) and then went "Guys! This is in 3D!" and didn't move on from there. Pretty much what they movie industry has been doing for a while now. (You think they would just get over it.)