I think people like variety too. I mean, suppose I feel like wearing a purple shadow, or a pink lipstick, it would be much easier to do so, if I owned the colors. So, here lies the question - when you say "collector", do you mean someone who purchases a lot of makeup with the intention of using it at some point, or someone who buys things and does not plan to use them?
If its someone who buys a ton of stuff they can never possibly use up, its probably about variety. I know I own way more clothes than I really need, but I like to have options. If I want to wear purple, I have purple ready to go.
If you are asking about straight collectors, well women collected Avon perfume bottles for years, and did not use the perfume, just because it was something pretty to look at. Some people like to collect stamps, just a hobby I mean what can you do with used stamps? And if you just collect and do not use it, no worries on expiration dates.
HTH!