Optimal memory layout isn't something you can know in advance. The optimal way to lay out objects in memory is exactly in the order that they will be accessed, but the runtime doesn't know that until after the user program has accessed them.

And if the program accesses a set of objects in different orders at different times, there is no one optimal layout.

I did ask Lars Bak once if they spent a lot of time thinking about cache usage and organizing objects in memory to take best advantage of it and, if I recall correctly, his answer was basically "no". They definitely think about it terms of packing objects into small amounts of memory. But in a dynamically typed language like JavaScript where every property is a reference to some other object elsewhere in memory, using the cache well is just profoundly hard.

Hell, it's hard even in Java where at least you do know the set of fields any given class has.