It can't. Coax can easily support Gbps connections (I used to have a 800 Mbps connection where the last few meters were over regular coax).
Now if you're talking about "why can't my copper twisted pair support that bandwidth", then it is a function of quality (the twisted pair is likely old, heavily bended, subject to interference, and running over old technologies.
Bandwidth is essentially a function of 2 things: spectrum availability (measured in Hz) and spectral efficiency (measured in bits/second/Hz).
For example, if a carrier "owns" 20 MHz of spectrum (let's say, from 700 MHz to 720 MHz), split in two symmetrical up/down channels, with a spectral efficiency of 4bps/Hz (a common value for LTE), and the cell towers have 3 sectors, each cell tower can provide 2043/2 120 Mbps up and down (although only 40 Mbps max per device, since a device can only be in one sector).
In wired communications, spectrum is less of a limit: every single wire has the full spectrum available. And if you are using fiber, then there is no interference, both from outside the wire or from different signals.
So air will never beat wired, because every single strand of wire has, in practice, the entire capacity of air.
PS: There are more things in play, such as attenuation, actual frequencies used (1 to 2 MHz has 1 MHz of spectrum; 1 to 2 GHz has 1000x more spectrum and therefore more capacity), etc.