While reading about a few cryptographic systems, I noticed that we always assume the communication channel is not secured. Why is this assumption made? And, why the effort is being put into designing cryptographic systems rather than working on making these communication channels more secure? I know the question is basic, but really cannot find a complete answer.
The goal of cryptography is to create these secure communication channels. However, keep in mind that a secure channel is more than just an encrypted channel. A secure channel should be able to provide confidentiality (using encryption), data integrity (using something like signatures), and data authentication (using something like certificates, MACs). The goal of cryptography is to create these properties to make a "secure channel" out of an unsecured channel.
Note that some cryptosystems build on top of other types of channels. For instance, in public-key cryptography, you generally need to assume that you have access to a authenticated channel for key exchanges (which is a stronger assumption than a completely unsecured channel).
If you already had a secure channel, you wouldn't need any cryptography. So if you're reading about cryptographic systems, you're reading about how to make things secure. Now, if you want to make a secure communications channel, the first thing you need is a communications channel.
Any time you transmit information you have have to send it through some kind of medium ( air, wire, optical fiber, ect.). The medium can be tapped into at any point by an adversary and is thus considered insecure.