Why don't brands do 3XL size clothing?

I guess they think that larger people don't care about their looks because otherwise they wouldn't be large and hence don't care about the brands. So it would not be profitable enough to produce clothes of that size. However, I do think that big people do care about how they look, they are humans too and humans tend to care about their appearance... Especially the people who are big because of some sort of malfunctioning of their digestion usually care about it. I'm the opposite, I'm underweight because of a digestion problem and it sucks that you have to go the the children's department to get a belt that works for you, I guess they should make clothes to smaller people as well ;)