[martincmartin]

The the object can be accessed (not modified, just accessed) from multiple threads, the extra instruction needs to be a thread safe atomic increment, which is hugely expensive, and becomes more expensive the more cores you have, especially in a NUMA architecture.

Also, when an object is freed, you need to decrement the reference count of every single object it points to, even objects that are referenced from many other places and clearly won't be reclaimed for a long time.