Because we want the makeup to do so many things.
Many of the ingredients are dyes and pigments to change the color of the skin or hair. Then we need something to make the colors easy to apply to the skin, so we add carriers and propellants. But we also want sunscreens to protect the skin from the sun, emollients and humectants to keep the skin from drying out, and we need preservatives to give the makeup longer shelf life.
We could use oils in the makeup, but those can cause acne, so we modify the oils by making esters out of the fatty acids, so we get an oil free makeup that does not cause the skin to break out. (See the lauryl benzoate molecule on the previous page for an example.)
Some makeups have both fat soluble ingredients and water soluble ingredients. To get these to mix and not separate like oil and vinegar dressing, we add emulsifiers to make something like a mayonnaise out of the watery and oily parts. An emulsifier is like a detergent – it is a molecule with one end that sticks to oil while the other end sticks to water.
A lot of the chemicals in makeup are perfumes.
Hexyl cinnamic aldehyde
Hexyl cinnamic aldehyde is the name for the scent in oil of chamomile, and it is one of the molecules that gives chamomile tea its aroma and flavor.
To keep the fragrances in the makeup, and to keep them on the skin, fixatives are added. One fragrance fixative is an ester of benzyl alcohol and benzoic acid, called benzyl benzoate.
Benzyl benzoate also has the side effect of being an insecticide that kills skin mites that can cause the disease scabies.