Why viruses are not considered as mobile code? Why worm is a type of mobile code?

Answer 1

Your question does not make sense to me. The term "mobile code" is usually used for programs written for mobile devices - smart phones, PDA, etc. There are some viruses that infect such devices. Those viruses are, of course, considered mobile code.

Also, worms are a kind of viruses. Remember, a computer virus is a program that can replicate itself. Worms can replicate themselves, ergo they are viruses. They are simply a subclass of viruses - viruses that explicitly use the network for their replication.