Most cosmetics contain water, plant extracts, proteins, or other ingredients that microorganisms love. Without protection, bacteria and fungi could multiply inside a bottle long before you finish using it.
The obvious solution might seem simple: just add a preservative.
In reality, it's much more complicated.
A good preservative has to stop unwanted microbes while remaining gentle on human skin. It needs to work with dozens of different cosmetic ingredients, stay stable during storage, and avoid changing the product's texture, color, or fragrance. Finding that balance has challenged cosmetic scientists for decades.
Global Chemical Database