Obesity is an excess of something. If we flip it, you can say that "too skinny is a problem" and there is a difference between someone someone with an eating disorder that makes them avoid food vs those who simply don't eat enough.
The unhoused has those people with a housing disorder, aka mental illness, and those who, simply, don't "house" enough.
Why don't they house enough? Many reasons. But nearly 4 out of 5 are not ticking the severe mental illness part. So there is less water in the argument that homelessness is caused by mental illnesses which is the leading reason I hear when people talk about homelessness. So, they aren't "mental," they are "normal."
Perhaps having a severe mental illness is somehow important for you but I still don't see how is it relevant in this context other than it shows that the fraction of homeless with it is ~4x bigger than in regular population so it's likely the rest of them are suffering with less than severe mental illness (not even taking drug addiction into account).