Hi Cue,
You kind of answered you own question, didn't you? Answer: Because they're "snobs". But you risk becoming one, yourself, when you label riders who prefer to stuff their jersey pockets instead of a bag as being "dumb".
Rarely can all riders be grouped into a single category and labeling one group as "snobs" is pretty shallow. There will always be people who want to look "cool" or "hip" or "pro" and will look down on and deride those who they perceive as not having the same image. But we shouldn't let that mislead us into thinking that all folks who look "cool" or "hip" or "pro" are doing it out of snobbery. They may have legitimate reasons.
Let me ask you this: How much do you ride? How fast do you ride? Do you ride just for pleasure or exercise? Or do you train to achieve a goal (like completing a century) or train in preparation for a race?
Depending on what you do with your bike, you may desire to reduce your and your bike's resistance to wind. That's what the pursuit of a low aerodynamic profile is all about. Riders who strive for this often abandon the use of bags because most of them have the opposite effect---they increase "drag" and increase their resistance to wind. In other words, they slow them down. Carrying their gear in their jersey pockets on their low back helps reduce that drag and lower their wind resistance---even if it looks goofy.
So there are legitimate reasons for their mode of thinking. They aren't necessarily being snobs. The fact that some are, is a different issue altogether.
My typical daily training ride is 35 miles so I rack up a lot of miles during the riding season. Depending on my goals, I sometimes carry an Ibera bag on an Ibera rack because I train for long distance rides and sometimes I need to carry more gear in case I need to do a repair without SAG support. I have a vintage Fuji road bike with a quad-butted steel frame for this. I'll also use it when I want the added weight and wind resistance to help me become stronger---especially when I ride against the wind as often happens along Lake Michigan were I ride.
But when I train for speed, I'll run the same course on my carbon Fuji which is much more streamline and I may not carry any bag. So it depends what I'm doing. I think you'll find that other riders mix it up, too.
Kind regards, RoadLight