Lots of people want to buy the pack first for two reason:
1. Its backpacking and you can't go without one. So, they think, lets get the pack and then put everything in it.
2. How do many people pack for vacation? They get out their suitcase and then start throwing things in it. That just get transfered to backpacking.

It take thought and experience or training to reorder the process to: figure out what I am taking THEN figure out what I will put it in.
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If I wouldn't eat it at home, why would I want to eat it on the trail?