Oh, I see, I always assumed because there were lots of international students in my uni including some that were there since little time. In fact I find this on the CROUS website:

La grande majorité des étudiants internationaux ne remplissent pas les conditions pour déposer un DSE (Dossier social étudiant) et ne peuvent donc pas participer à la phase principale. Si vous êtes un étudiant international poursuivant son cursus de formation en France, vous pourrez déposer une demande de logement Crous dès l'ouverture de la phase complémentaire (9 juillet)

So it seems international students can access CROUS housing, but they won't be prioritary as they have to sign up on the second wave? At least the stipent is a sure no except after 2 years.

You're right to say the CROUS housing is insufficient... In fact all of Paris-Saclay (what I assume you mean by UPS) has insufficient housing, they built the labs, moved the schools, and expanded the university before building any housing... It took 2 years from the start of my PhD at a lab there to see the first housing, restaurant or supermarket that you didn't have to take the car to go to. And my lab (and others) had been there since a few years (like 5, some much more) before I arrived, too.

They decided to build one of the world's largest academic centres in the middle of nowhere (in a way, literally, as this stems in part from CEA Saclay who put itself there to isolate the population from possible nuclear accidents while not being too far from Paris), and housing was a second thought.

However, in my opinion, this is the result of incompetence rather than malice, as I find this a very inefficient scheme to send money the landlords' way, they could just increase APLs (the CAF you mention) or outright subsidize construction or private student housing.

And even without international students, the situation would be the same there, as the uni naturally attracts from all over France, and it would probably never be lacking candidates to fill the classrooms (thus the studios and rooms).

In fact it's terrible for professionals too, who don't compete with students for housing. Well, actually they do, because that's how expensive things are, even people with wages live like students there, or 1hr away. That was the case of my PhD advisors, I really do not envy their life, even if the work and campus is very stimulating.

France's absurdly elitist system of the prépas and écoles eviscerate the universities

Don't get me started on that! The most ironic is if you mention Paris-Saclay or Sorbonne on one hand, and Polytechnique or ESSEC on the other, outside of France, most people will likely know the former but not the latter. Polytechnique was too good for Paris-Saclay by the way, in which they were initially, but eventually bailed from to make their own Paris Saclay (Institut Polytechnique de Paris).