Many of them would love to do that. Jibs are not always easy to find, especially when you have a limited skill set or education. Others do have jobs. It is just the job doesn't pay enough to cover basic housing. Others have mental illness and cannot hold a job.
lack of experience
The area they live in
The cost to get clothing to interview for job
The lack of jobs
Not having a permanent address to start with, most employees require it.
Some do not want to be part of the treadmill of working for the majority of their life, dying or getting Alzheimer's and then having their home sold to used to pay costs for a care home. In total ending up with zero for all your hard years of labour.
All the previous answers are correct. In our area the majority have emotional as well as mental handicaps. I do remember once while visiting California I noticed wooden platforms erected under highway overpasses. I discovered men and women actually slept there over night. In the morning they would walk to a train station where they had their personal possessions and clothes locked in a large bike locker. They would use public washrooms or wait to get to work and shower and clean up. They had cell phones, laptops etc but no permanent address. They worked in all sorts of businesses and used P.O. Boxes or friends addresses for mail etc. They had decent salaries but could not make enough to afford housing.
There is no 'one' answer..in fact there are many different reasons, most very complicated.
There is a very high percentile that can't get a job because, of things out of their control like mental or physical illness and addiction. Most of those have fallen through the cracks in our so called sophisticated society. Some are run aways and are victims of abuse of one sort or another, and simply have no place to go. Some, because of bad choices, and lack of support have lost what little they did have, and have no where else to go.
Many of them don't get a job because they can't. Others don't even bother to get a job, but that would be a minority. They're just like any other people. There is a very heavy bias on homeless people, and this makes it really hard for them to get a job.