While luxury condos don't "make" homeless people homeless; they're just a graphic depiction of how flagrantly housing prioritizes profit over the actual use value of housing people. And they could make homeless people not homeless if we had a sane system.
As for support for homeless people, I agree that it's tricky and we shouldn't just shove them somewhere, but it's not insurmountable EXCEPT when systems for supporting people are systematically defunded and people are kept without the support/job/community they need. And that's the case everywhere. It seems much harder than it is because it's impossible to solve within unrestrained capitalism, which is the natural state capitalism slides towards all the time.
As one final point, it isn't inefficient in any real sense to have people live in cities. In fact, with reasonable infrastructure (admittedly nonexistent in the US), it is much more efficient in terms of any real resources for people to live densely, especially if they need support. The only way in which it is inefficient is that landlords and profiteers siphon wealth away and drive costs up to the sky. This is a solvable problem but it is a problem of capitalist market anarchy, just like cities busing their homeless away.