Also, there are a lot of jobs that are insecure, poorly-paid, and don't give people a chance for advancement.
So leave, get a better job. That's a life decision. Nobody is forcing anyone to stay working for a company who doesn't even offer job security.

Mental illness can be another reason.
A lot of people with mental illnesses are successful.

A lot of homeless people are those who fell through the cracks of the mental health system and can't hold a job for reasons that aren't their fault, unless you think schizophrenia is a life choice.
My brother has schizophrenia. Maybe it was the psychedelic drugs (life choice), maybe it was the way his brain is wired. idk. But what I can tell you is he's not what I would class as poor. He gets full welfare, basically double what I got, maybe more. I never asked him, none of my business. But legit mentally ill people get probably a minimum of £20k a year. That's basically a full time salary at minimum wage. I don't consider that to be "poor". That's enough money for a comfortable survival if you're careful with how you spend money. £30k is about the national average, anyone earning that is certainly not poor.

I'd say any adult earning less significantly less than £20k a year is poor. People who earn more but struggle to make ends meet, maybe they're not making good economic decisions. How many poor people in the UK do you suppose have a smartphone?

A person with an 80 IQ is not going to become a banker.
Probably not, but our society doesn't just reward intelligence, it also rewards hard work. And that doesn't just mean physical. Worst case scenario if you really can't get a decent job is you get £20k at minimum wage. Ok no holiday in the Maldives but enough to get by. More than what most people in the world have, and 66% of what the average Brit has.

or because he's a sociopath who is happy to make a good living by scamming people, e.g., estate agents.
If you think estate agents are scamming their clients, then go into business as an estate agent that doesn't scam their clients. Presumably you'll be charging significantly less for your services, so you'll have no problem getting customers. Capitalism baby.