[motorest]

> The point is to optimize data layout for access patterns.

Yes. That's the point.

> Putting fields of a struct into their own arrays is only actually an optimization if you're only accessing that field in-bulk.

Yes, that's the scenario.

> And if so, why is it even in a struct in the first place?

Because that's how everyone is taught to model domains.

> If you use all fields of a struct in your algorithm, then an array of structs is the optimal way.

No. Your personal belief goes against both theoretical and empirical evidence. Others already talked about cache, padding, vectorized instructions, etc. I recommend you do a quick googling on the topic.