I think your model of open/closed is incomplete and thus misleading. There are more states to a process than "active" and "inactive," and it is not optimal for the system to simply move processes between those two gross states. The obvious example is non-foreground apps during multitasking. A less obvious example is an app during a background refresh.

"Fully closing" a process is not necessarily cleaner than letting the system allocate intelligently, despite what one's puritanical upbringing might make them believe. (Consider how artists often need a messy space to optimally hold their processes.)