Do you really not understand?
I've been homeless 4 times as a result of my own mental illness
I have ocd primarily but at times I've experienced psychosis, hallucinations, panic attacks, severe depression that have made it literally impossible to function - as in cannot string 2 words together as a THOUGHT let alone speech and unable to process actions as simple as walking, eating...
Incredibly debilitating AND there is very little support what mental health services there are, are HUGELY overwhelmed, underfunded and under resourced.
Training in Mh for hcps is woefully inadequate- I've been assigned cpns with NO experience in dealing with someone with severe ocd and do more harm than good, I've had meds chopped and changed at the whim of gps, I've been refused support because I've been lied about BY hcps who didn't like that I knew more about my condition and how it affected me than they did
Certainly landlords and councils and housing associations barely understand mental illness but they're also not getting support to do so or funding to allow them to support the mentally ill.
When very ill I've been cognitively unable to manage money, too anxious to even check my bank balance and have run up debts when I wasn't totally in touch with reality.
Family and friends IF they stick around (most don't) also don't have the training or expertise to help and may be ltd in what help they can provide due to their own circumstances.
I've been VERY lucky (so far) that I haven't been street homeless through a combination of having a child in my care when homelessness hit (they mostly let me keep care of her except when I was at my most ill) and being able to "sofa surf" with certain very kind, very understanding loved ones, forever grateful to those people.
It's terrifying - and it's not that unlikely to happen to ANYONE you never know what's around the corner. Mental illness can hit anybody without warning. Most frightening example I knew of was a mother in a post natal psychosis stage, power cut happened which freaked her right out and she panicked about how to keep baby warm and almost put him in the gas oven (seemed perfectly logical to her at the time) luckily partner and baby's dad got home just as she was about to do so, no previous signs she was this far gone, and stopped her and got her the help she needed. She now has no memory of this event in experiential terms she knows it happened cos she's been told but she has no memories of that week (roughly) she was so ill
People forget their meds, or have trouble getting hold of them due to mix ups etc and with some conditions missing even one or 2 days worth can send them spiralling.
A friend of mine with bipolar disorder got caught out with a bank holiday/repeat prescription mix up and ended up on a manic "high" that whole weekend that caused her to spend all her money and then some in a casino! She isn't a gambler at all usually this is just how it affected her that one time, couldn't make the rent once she was on an even keel and was VERY lucky that her mothers church stepped in there (she paid them back when she could) or she could very well have become homeless if she didn't have that support
It really just takes a series of bad luck...
Can happen to anyone