Here's why. It isn't like to old days, at least for some cars. New keys have computer chips in them. These chips speak with a computer program in the car. Only when the chip and computer program agree will the car start. It is an antitheft method. It's clever, but it doesn't come cheap. You can't replace a key at your hardware store. It has to come from the dealership. You have to take in the car, ask for a replacement key, hand over any remaining keys and the new key, the old key(s), and the computer program are all adjusted to agree with one another, ensuring that your shiny new key will start the car.
All of this will run you somewhere in the $140 to $160s. Better to keep tight track of those keys your car came with.
And that's the five minute response!