Same with software bugs it's always "careless" programming.

My first mentor immediately took me to task when I called something I had done "a stupid bug." He stopped me and said "All bugs are stupid[1]." The point being that the overwhelming majority of the time, once you are looking at a bug, it looks like carelessness and stupidity, and yet very smart, careful programmers introduce such bugs with surprising regularity.

1: The motto is "All bugs are stupid" but he did admit later that there are such things as "smart bugs" though they be rare enough to be a rounding error.

This is true, we have the ability to house every homeless person. There are more second vacation homes than homeless people. Why not just move them in? I'm not even talking about the ones available for rent - just the ones sitting there empty.

As far as I understand, we do have the physical capability of housing that many people, but a significant number of the homeless are drug addicted, have mental illness, or both, and attempts to provide housing in mass like that leads to many of the houses becoming filthy and/or damaged. And who would want to give up a house for that? Homelessness can't just be solved with a housing band-aid, but needs to be addressed at the source with support for the drug addicted and mentally ill.

Having safe, stable shelter would seem to be a prerequisite to helping people deal with drug addiction and mental illness.

Baxter, A. J., Tweed, E. J., Katikireddi, S. V., & Thomson, H. (2019). Effects of Housing First approaches on health and well-being of adults who are homeless or at risk of homelessness: systematic review and meta-analysis of randomised controlled trials. J Epidemiol Community Health, 73(5), 379-387.

We should build enough shelters for homeless, but separately for those who are addicted and those who are not. (Maybe more than two types of shelters.)

For a homeless person who is not addicted, sharing the shelter with the addicted ones means constant worry that even the little stuff they own gets stolen the moment they stop paying attention, and also lack of sleep because of the constant screaming and fighting during the night.

That is one factor, but it isn't everything, there are still other factors so don't overstate it

As someone with experience with severe mental illness (BP1) I don’t think it’s possible to overstate the importance of a stable environment when dealing with an acute mental health crisis.

downvoted because they clearly said 'pre-requisite' which is in no way an overstatement.