I probably would be a psych case as a collector of Barbie items! LOL
I know why I collect certain things though and I'm pretty sure it is psychological for me.
When I was a young child, I was taken from my mom. She married a man when I was 4 and he abused me. They had a child together, my half-sister. After my mom left, I went to live with my grandparents. I was expected to grow up way before my childhood was over. I was taught to hold in my feelings. My grandfather didn't treat me like this, but the women role models in my family did. I had very little toys even though money wasn't an issue.
The day I was taken away, I left with nothing but the shirt and pants on my back. I probably had shoes on. All of the toys my sister and I shared were left behind. Including my favorite birthday Barbie with a sparkly dress.
Years later, I'm all grown up. When I found out I was having a daughter, I started a Barbie collection for her. It may seem ridiculous to some. But I guess it's a way of preserving what was taken from me in a precious time of life.
I can't speak for others though. That's just why I am a collector.