Why don't we see all seven colors of a rainbow?

Every color imaginable is in a natural rainbow. The problem is that when you are far away, they all blend into one another, so we the primary colors (Red, blue, and yellow) all blended.

well usually i see all of them, but even if there are 2 people standing shoulder to shoulder you dont see the same rainbow. it depends the way light is refracting through rain drops. no two people see the same rainbow

Speak for yourself.

u should look more clearly... maybe u cant distinguish between them

As Francesca writes, we see all the colours because the sunlight contains contains them all. But depending on the background and the angle of the sun over the horizon, it may appear differently at some occasion. The sunlight contains all the electromagnetic frequencies. Our eyes have receptors sensitive to three different colour frequencies: Red, green and blue. Colours that you see directly are called additive colours and the sum of them is white. Colours that are reflected from a surface are called substractive colours and the sum of them is black. With additive colours such as sunlight or the light from your monitor or TV, a mixture of the three basic RGB gives you the intermediate colours. For example, yellow is a light that has the red and green frequencies in it. There aren't 'basic' colours or 'seven colours' since frequencies are endless. It is just a human convention to define colours with names and subdivide them with e.g. "light browish red."

Between each colour the colours merge so you have Red then red/Orange then Orange etc... You will pick out the most prominent colours with your eyes - people have different eyesight so someone next to you may pick out different colours to you as more prominent. They are there - you just have to look between the bands of colour.