Why do red, green, and blue make up all the colors? Why do red, green, and blue combinations can make up all the visible colors?
Let's reminds ourselves what light is.

Radio waves, micro waves, X rays and gamma rays are all electromagnetic radiation and they only differ by their frequency. It just so happens that the human eye is able to detect electromagnetic radiation between ~400nm and ~800nm, which we perceive as light. The 400nm end is perceived as violet and the 800nm end is perceived as red, with the colors of the rainbow in between.

A ray of light can be a mix of any of those frequencies, and when light interacts with matter, some frequencies are absorbed while other might not: this is what we perceive as the colors of objects around us. Unlike the ear though, which is able to distinguish between a lot of sound frequencies (we can identify individual notes, voices and instruments when listening to a song), the eye is not able to distinguish every single frequency. It can generally only detect four ranges of frequencies (there are exceptions like daltonism or mutations).

This happens in the retina, where there are several kinds of photo-receptors. A first kind, called "rods", detects most frequencies of the visible light, without being able to tell them apart. They are responsible for our perception of brightness.

A second kind of photo-receptors, called "cones", exists in three specializations. They detect a narrower range of frequencies, and some of them are more sensitive to the frequencies around red, some to the frequencies around green, and the last ones to the frequencies around blue.

Because they detect a range of frequencies, they cannot tell the difference between two frequencies within that range, and they cannot tell the difference between a monochromatic light and a mix of frequencies within that range either. The visual system only has the inputs from those three detectors and reconstruct a perception of color with them.

For this reason, the eye cannot tell the difference between a white light made of all the frequencies of the visible light, and the simple mix of only red green and blue lights. Thus, with only three colors, we can reconstruct most colors we can see.

By the way, rods are a lot more sensitive than cones, and that's why we don't perceive colors in the night.