A German student visa allows you to work 120 days per year [0]. At minimum wage [1] that is about EUR 9,000 per year. I think there would be many international students who could support themselves with that. Standard of living would be low compared to Germans, but it's possible.

You do however need to show you have EUR 8,640 before you get the student visa [2]. Very poor families would not be able to borrow this amount, so it acts as a barrier for them.

[0] https://www.daad.de/deutschland/in-deutschland/arbeit/en/914...

[1] https://www.google.com.au/publicdata/explore?ds=ml9s8a132hlg...

[2] https://www.daad.de/deutschland/nach-deutschland/bewerbung/e...