Question: Like ethernet cable, optical fiber. We have a square wave signal, do we just send that squre-wave signal directly down along the cable? If we do send it directly, why we can not send the square-wave signal directly into the air wireless channel? What is the reason behind? Is it because the cable or wire are more stable but wireless is very unstable?

Answer: 
Along with all the bandwidth and other considerations mentioned, the main reason you don't transmit your data directly is that **everyone** would be transmitting at the same frequency.

Take [Ethernet.](https://en.wikipedia.org/wiki/Ethernet) It uses two (or more) pairs of twisted wires to carry two separate channels of information (your Ethernet card sends data on one pair and receives it on the other pair.)

If you just directly broadcast the signals from each pair, then you couldn't separate them.

The wires confine the signals do that they go pretty much only where they are intended to go.

If you just modulate each onto a carrier, you can separate them - but then you have know which carrier frequency is used for what.

That would work for a few channels, but would get difficult really quick as you add more channels (users.)

WiFi gets around that by having multiple channels and using some intelligence to decide who uses which carrier frequency for what and for how long. It also has WiFi routers which coordinate the end users and actively tries to avoid interference from other routers and users.
The voltage or current you apply at one end of a pair of wires can be detected directly at the other end. It's like pulling on one end of a tight string -- you can feel the pull at the other end.

When communicating wirelessly there is no quality of the source that can be directly felt or measured at the other end. It's like you're standing in one end of a pool. You can hold your hand at any height and the water at the other end is unchanged -- the height of your hand is undetectable. If you move your hand up and down rapidly, however, you can make a *wave* that propagates to the other side of the pool where it *can* be felt and measured.