Well, I'm not sure that hundreds of different people are ordering these cartridges every single day. Hundreds would mean a minimum of 200, and every day is a minimum of 365. That would work out to 73,000 separate orders every year...
Also, the very same dynamic that you note - the ability of the internet to make all kinds of previously hard-to-find items easily accessible - makes it perhaps too low a bar for something being truly common. All kinds of things that are fairly rare and nichey (not just gun stuff - any area) are readily available online.
Again, just because the internet will let you ship all kinds of exotic foods and drinks to your door (perhaps packed in dry ice) does not make those items as common as McDonald's.
Now, where you and I would likely agree is that it rarely makes sense to limit oneself just to the "common" or ubiquitous cartridges if one is otherwise interested in something else. Yet some people very vocally insist on just that approach. To each their own.