Well, a lot of creators are actually doing amazingly well on Patreon, but I do get your general point.
I'd say it's because comics have always been and will always be niche media. People may know the famous characters, but only a tiny percentage of those people have ever actually picked up a comic. And a minuscule number have ever paid for one.
With Patreon, you're asking someone to pay for something that most of the time they can read for free, so that's another hurdle to overcome, but really, I know with my Patreon habits, I tend to support people who I think are doing important cultural work, and I suspect a lot of people are doing the same. Comics are great, but they're mostly just entertainment. It's not as if I think, "okay, if I don't support this comic creator, he might stop, and society would be worse off because of that."