Because your brain is separate from all the other brains, "you" as well are separate from all the other people. You are similar but separate. Two things could be the same but also be separate. Take two apples. They are same, they are both apples that look exactly the same, yet they are still separate things from each other. Just because two things are similar or same, doesn't mean that they are one unified thing, they could be both the same things and be separate at the same time.
Opposite of same is different.
Opposite of separate is unified.
A couple of monozygotic twins could look the same, talk the same, act the same, carry the same genes. They could be extremely similar or even the same. But just because they are same, doesn't mean that they are unified. They have separate bodies, separate compositions, separate nervous systems, separate brains and separate streams of conscious experience. They might be the same but that does not make them unified, they are still separate beings in any regard.