Because simulations do not have the properties of the things which they are simulating. They are a mathematical representation. Not an actual manifestation.
I guess it depends what you think consciousness is. If a computer works out that a conscious being would do a certain thing and simulates them doing so, versus a material being undergoing the same process and performing the same act, is there a meaningful distinction between the two? I guess you could say the "real" being "feels" it whereas the simulated version doesn't. But what does "feel" it mean? Once again it is a curious situation where in order to preserve a materialist perspective on reality one first needs to insist that there is more to reality than the material. That the mental world has a reality beyond what can be explained purely by the working of its parts.