For one, if you're carrying enough crap isn't a backpack more ergonomic (assuming you wear both straps and aren't one-strapping it like the cool guy you are)
"Vestis virum facit", or clothes make the man is extraordinarily powerful in business and social dealings. Clothes signal a lot of information quickly. So this isn't about you caring what other people think, it's about how what other people care about affect your options. I could prescribe a few experiments to witness the differences directly but I'll tell you from personal experience that the A-B differences are stark.
There was a time when I was younger that I raged against this rule, but it's a youthful variation of "old man yells at cloud". You can try and create a new reality in your head and believe it fervently, and cut your nose to spite your face, but image matters greatly to other people, at a deep level.
If I need a job done, I couldn't care less if the person who is able to do it wears a backpack or not.
FWIW I think messenger bags are shitty design and think people who use them just don't know any better.
There's an auxiliary benefit as well in that a backpack seems to be way better for my back than a shoulder bag.