They are proprietary but they don't persist on your computer when you clear cache and close the browser, unlike apps that can linger and spy on you forever, eve

They are proprietary but they don't persist on your computer when you clear cache and close the browser, unlike apps that can linger and spy on you forever, even after you supposedly uninstall. Uber famously got in trouble with Apple over that, but who knows how many are doing it without getting caught.