I'm scratching my head at how someone could conclude that poverty and homelessness don't really exist in our societies just because he or she hasn't experienced it first-hand.

Besides, most people that are truly homeless, ie really have no permanent place to keep their stuff for a significant period of their lifes in the US are homeless for the same reasons as the homeless in France or anywhere else in the western world, and that drug abuse, alcoholism och mental disorders. That is not a poverty issue.

OK, first:  "Truly" homeless?  Are you really arguing that a person is not "truly" homeless if he or she only lacks a home for, say, two months?  Two weeks?  Two years?  What kind of "significant period" meets this mysterious defintion of "truly"?  Just out of curiosity.

But defining only the chronically homeless are "truly" homeless is a handy way to pretend that poverty and homelessness aren't real societal problems that need to be addressed.

Second, your definition of "most" needs some work.  According to the National Health Care for the Homeless Council:

Approximately one-third have mental illnesses. Perhaps one-half have a current or past drug or alcohol addiction.

That is not my definition of "most."  It is, of course, more convenient to believe that the only thing that could possibly lead to "true" homelessness and "true" poverty in our enlightened societies is mental illness, but that's just fantasy.

There are many factors that lead to homelessness, including domestic violence and illness - and for the record, I'm talking about illness of the physical kind, since it seems that some people believe that those who suffer from illness of the mental kind are for some reason less deserving of sympathy or support.  Which is not a belief I share, but let's move on.

Next, mental illness and drug addiction are issues of poverty, in that the poor and homeless have far fewer resources for dealing with those problems than the rich and homed.  Diseases of the physical and mental kind affect the poor and homeless in roughly the same proportions as the general population.  But the poor have fewer (or zero) treatment options, and are likely to encounter much greater difficulty getting help.  As a consequence, they may not recover from illnesses (mental and otherwise) that a person with more resources might recover from easily.

What's the result of all this?  Let's just talk about my hometown, the so-called "Capital of the Free World."  In Washington, D.C., according to the Washington Legal Aid Center for the Homeless, nearly half of all homeless people are women and children.  One of the largest homeless shelters in D.C. is run by the CCNV:

Over 65% of the shelter guests work full- or part -time on a regular basis.

That's right, they're working full- or part-time, and are still homeless.  They're living in a homeless shelter, not a military barracks.  This is genuine poverty and true homelessness.  It's real, and denying that won't make it so.