You could argue that objects aren't deallocated in non-GC languages either, the memory they had allocated is just available for other objects to use. Which sounds kinda like "deallocated" to me, and yet it's the exact same thing as the allocation pointer in a stop & copy GC being free to overwrite them.
Nor is it particularly desirable. One of the cornerstones of how a modern generational GC performs as well as it does is that blocks of objects are freed up all at once.
If they were deallocated in a piecemeal fashion then:
- Freeing up memory would take longer, because you can't just change a pointer back to the root of the generation.
- Allocating memory would take longer, because it involves searching for a big enough empty slot rather than just throwing the new object on top of the heap.
- More memory is wasted overall, because of heap fragmentation.
