Google started in the USA. In the USA if you give feedback, no matter how well-intentioned or apparently harmless it may be, that can be used to sue you. Even if those lawsuits are unlikely to go anywhere, they are a distraction. So no US company will give you feedback.
And that's become standardized in Google's procedures.
The problem is that a certain fraction of applicants will become convinced, rightly or wrongly, that they were hard done by, and were blackballed for whatever reason. Any hard information you give them can be used against you.
And unfortunately not just obvious stuff like "we didn't hire you because you're black". You can say something innocuous like, "your grasp of algorithms didn't seem strong enough" and the candidate will flash to a particular interviewer who asked an algorithm question they didn't get, and become convinced that that was who blackballed them. And can further become convinced that that person actually had it out for them for unrelated reason X.
This might be unlikely to wind up in court, but it happens often enough that it shows up in nasty blogs, rumors, etc that every large company has learned that you just don't want to go there. And none of them do.
Google started in the USA. In the USA if you give feedback, no matter how well-intentioned or apparently harmless it may be, that can be used to sue you. Even if those lawsuits are unlikely to go anywhere, they are a distraction. So no US company will give you feedback.
And that's become standardized in Google's procedures.