Why can only analog signals pass through air (wireless channel)?

Adding to Tom's answer:

The wording is not very clear, but what this means is that digital signals do not actually exist in reality. All signals are analog.

When we decide that a voltage above a certain threshold is a "1", a voltage below a certain threshold is a "0", and the space in between is "undefined", then we interpret an analog signal as a digital value. However, it is only a very convenient approximation that greatly simplifies the job of the designer.

Digital is abstract information. It is a meaning we choose to assign to physical values. This is why you cannot send a digital signal over the air as radio waves. It must be converted first into something that exists outside of abstraction, like an analog signal which represents the information to be transmitted.

The real signal is made of physical analog values: voltage, light, current, fields, acoustic pressure, whatever.

For your radio application, you could encode your digital bits into the frequency of a carrier, or its phase, or any other encoding, of which they are many. Now, you have an analog signal which carries your information, and you can transmit it, then receive it and recover your bits.

The important take away point is that you need a continuous waveform if you are using electromagnetic waves. That is not to say you can't have a signal which represents digital data, just that the signal itself must be continuous.

Consider a square wave, or even a sequence of binary voltages (1 0 1 1 0 etc.). If you take the FFT of such a signal you will find that it has spectral content over an infinite bandwidth. In other words, to produce a perfect step change you need a channel with infinite bandwidth.

There is no such thing as a channel with infinite bandwidth. In the case of sending electromagnetic wave based signals wirelessly we have a massive limitation on bandwidth which prevents a non-continuous waveform (i.e. ones with step changes) being sent.

However just because you can't send a signal which is non-continuous, doesn't mean you can't send a signal which represents one. All of the digital modulation schemes do just that. OOK is the most basic example - a zero is represented by no signal, a one is represented by a simple tone.

Digital signals are an abstraction that humans use to describe and understand things by omitting information that we are not interested in.

For example, consider the following: 1001010101000101010

Is that a digital signal or an analog one? If you only care about the pattern of ones and zeroes, then it's a digital signal. But the actual physical thing you are looking at is entirely analog because each digit is in a slightly different physical position and has a slightly different level of brightness and so on.

There might be exceptions in quantum mechanics, but that's not relevant here.