I feel like, "why can I buy parts at retail when my lbs can't get them at wholesale" is very close to a self-answering question. They can also buy parts at retail, but that doesn't exactly pay the rent, does it?
I saw the owner of my LBS buying parts from Amazon. That guarantees they lose money on those parts. I know over the last couple of years, they have spent an inordinate amount of time trying to find parts and it's not always possible. They were trading parts with people. There has been some indication that big mail order businesses were getting parts through normal channels while small shops couldn't. It has been a difficult time for most bike shops.