Because objects in these languages don’t have a uniform representation, it is up to the implementation how a java object is layed out in memory for example. But an allocated memory region is just a.. memory region in java and in C++ as well - you can store your custom data structure in both.

Sure, value types will be a huge win, but it is not like you can’t store your RGB class’s data quite trivially into a huge linear structure as SOA or AOS as needed, and deserialize as needed.