It's the economy of scale. When you're making $4k a month on popular crowd-sourced projects, an extra $200 for a commission is probably only enough to cover the taxes you'd have to pay for it. Especially when that commission is going to cut into the time you normally budget for your regular projects.
It's just not worth it. In fact, it might cost you money if you charge too low. It's not right or wrong to say that you'd have to charge an arm and a leg when doing a commission would otherwise be, in terms of your business, literally not worth your time.
I'd argue the opposite. When you're keenly aware of how much your livelihood depends on people enjoying your stuff, there's pressure that comes along with that. It's true that you're probably less concerned with individual revenue streams, and therefore pleasing any individual supporter, but one of the pitfalls that comes part and parcel with systems like Patreon or Subscribestar is that you have to do what is popular instead of what you might really want to do.
I'm sure there are some creators who would like to do more stuff for niche games like Detroit: Become Human (Kara is bae) or Elden Ring or what have you, but the simple fact of the matter is that Claire Redfield, Tifa Lockhart, Princess Zelda, and others like that are just more popular and more marketable. If you're making content like that, you literally can't afford to work on niche passion projects. Not often and not consistently, anyway.
It's a shame, but that's just the way it is.
