Every object requires a header and a 8 byte pointer. This means even an empty object will cost you 16 or even more bytes. If you have perfect data locality and zero cache misses then you will still suffer from exhausting your bandwidth on useless data.

Then there is the fact that you still cannot actually control the allocation. The allocation pattern ABBAABABBA has worse performance on read accesses of all As or all Bs than AAAAABBBBB but with two arena allocators you can write your code to allocate memory according to the irregular pattern but still have continguous data as the end result.