Well, you can sign with a fake computer signature, but never you real one. I actually have a computer one that looks almost like a real signature... some nice bait.
Here are two reasons I can think of.
1) Think of all the contracts that will magically show up with you 'signature' on them for something they didn't previously have.
2) Do you trust a CA to secure your information? Identity theft is already an issue since they do very little to secure the information they do have.