IMO, it's one of four primary reasons, like primary colors....
1. Control. People want to maintain some level of control, and that means controlling your bags/belongings.
2. Fear. The fear of losing your bags entirely or not having them when needed.
3. Impatient. Wants the bags now. This is me. I am the guy looking out my door every time I hear a noise outside. Impatient people just do it themselves, it's faster and easier (they tell themselves)
4. Smugglers. Those trying to slide on a little contraband, like blue-green colored vodka in a 2 gallon Scope bottle for a 7 day cruise.
I am sure there is some mix and match there, but like the primary colors you can probably build every reason from there four base primary reasons.