The point is that a simple SoA implementation requires this - each field in the monster struct is an item in 20 different arrays. So, removing one monster means removing that item from those 20 arrays.

Now, as others have suggested, you can have a more complex implementation, where instead of removing the monster's fields from those arrays, you just mark them as "dead" or whatever and then skip them when consuming the relevant arrays, with some relatively small extra bookkeeping overhead. Of course, this comes with its own drawbacks, especially if the number of monsters is very dynamic and you are memory constrained.

The point is not to say that SoA is never good for performance, it obviously and certainly is, probably even in most cases. It's just not always best for performance, this was all.