I have a glasses prescription, why can't I use it to order contacts?
Glasses prescriptions describe the lenses required to correct your eyesight when mounted in a spectacle frame. Contact lenses are worn on the eye, and are shaped to fit the eye, and for that reason the prescriptions for glasses and for contact lenses are different. In fact, there is no scientifically accurate rule for the optician to predict either one accurately based on the other.
Contact lens prescriptions don't just describe the power of the lenses, they also describe the size and the shape of the lenses, so they fit your eyes properly. If you wear lenses that don't fit properly, either your vision will be rubbish, or else they will stick to your eyes and you will end up at A&E having them peeled off.
Plus if you've not worn contacts, getting contacts for the first time's a bit like getting measured for a dress or a suit, it might take a few fittings, plus you need to be taught how to take the lenses out (and put them in) and you need advice on how to wear and care for the lenses safely.