When the platform's native data structures are explicitly managed but not garbage collected, the language runtime can't just steamroller them, it has to notify the platform about each individually becoming disused. It's the worst of both worlds, you pay for O(n) work to handle garbage as well as not reclaiming memory immediately, and it's worse with an API that takes graphs of first-class objects rather than buffers which happen to contain the bits you want (which are incredibly risky, but the language runtime can handle that once and for all).