Cosmetics are unique because they must be highly visible in their packaging while remaining highly secure. They can't be obscured by their packaging, but because they are generally small and high-value, they must be tamper-resistant.
This is why plastic packaging is generally the standard for cosmetics—it can be extremely tough without sacrificing clarity. Of course, this means that the designer must know which type of plastic provides the best blister packaging solution for the job. Certain plastics are highly shatterproof, but scratch and compromise the product's aesthetic appeal. Others may be strong, but risk losing their clarity during manufacturing. There is no room for error in packaging for cosmetics, so it takes a designer with encyclopedic plastics knowledge as well as experience in this particular industry.