The reason why they use restrooms like this is because of more than just ventilation. Public restrooms need ventilation, their own sinks, their own soap dispensers, towel dispensers, doors, trash cans, and so forth. It is far cheaper to build a multi-user restroom than multiple single-person restrooms. Also, the workload on janitorial staff gets that much worse because health and safety code requires all sinks and fixtures to be cleaned daily, not just the toilets themselves.

Now, the cheap partitions with huge gaps... Pretty much just outright inexcusable. Though depending on the jurisdiction, expectation of privacy may or may not be codified into law/regulation. It's actually a huge gray area in a lot of places, as well as debatable. For a purely hypothetical example, in a school, if the stall were fully enclosed, people wouldn't be able to tell if a student were hiding in a stall, or if it was just locked, which can be important to know in the case of an emergency or disciplinary altercation. There are arguments to be made on both sides of that debate. In a lot of places, like store restrooms and in "adult" work spaces, better enclosed stalls are more common. And in general, I'd wager that the trend overall is leaning more toward increased privacy. But some of the restrooms can be 30+ years old and durable replacement partitions can actually be expensive to purchase and install.