Because you are not multiplying the number by anything. It's like multiplying 2 x 1.
I hope that made sense ...
It is similar to an expresion just break it down x*2 is 5 nothing that is a whole number will complete this equation so the answer is 2.5
if we take a peek at the graph, the y-coordinate at x = -4 is y = -3, and the y-coordinate at x = -1, is y = 3,