How come? They are part real numbers, what would you call the other part?
Usually they explain it something like: oh, at first people didn't know what 2-5 added up to, but then we invented negative numbers. Well, complex numbers are that but for square roots of negative numbers.
But that's a completely misleading way to explain these things. Complex numbers aren't numbers aren't numbers really.
As one broadens the idea of what it means for anything to be a number, we acquire/invite new members to the family and with great benefit.
If the square root of -1 is not a number, what is it? How come you can do arithmetic with it?