Except that it might be, because if people knew how to do it, they would have for the past 20 years that ECS have been implemented in the games industry that is notoriously focused on memory locality optimisation.
(not that I'm advocating for the patentability of software in general, I just think that if software should be patentable, this sure seems like something that could be)
Because "group entities together in memory by which components they have" isn't patent worthy.