Each system tends to operate on different sets of data which don't tend to be very similar, and systems may operate on multiple data sets. You could build each

Each system tends to operate on different sets of data which don't tend to be very similar, and systems may operate on multiple data sets. You could build each system as an object, but you wouldn't want to store the relevant data structures within that object because other systems will likely need to use those data structures as well. The entire architecture is predicated on separating data and behavior. Yes you can build an ECS system using classes, but nothing about it fits into what you'd call OOP.