The DB is correct, coaxial cable isn't equivalent to open air or a vacuum therefore the RF travelling through it is delayed by the surrounding medium of dielectric, the speed of which is designated by the term Velocity factor (or VF for most who have frequented this forum for any reasonable length of time).
The VF is a percentage of the speed of light within which the cable allows RF to travel. 66% is typical for RG-8/213, meaning, it requires more time for RF to traverse a distance of coax than it does that same distance through the air or within a vacuum.
Even aluminum in air has an average VF of .9979%, just a little slower than 100%.
Inside 66% VF coax it requires 150% the time for RF to travel a given distance it does outside in open air or vacuum.
If a half wavelength is 18' in air then it is 12' inside/ through 66% VF coax.
Your post should've been a question of 'Why 12', or 'why a 1/2 wave'?
A half wavelength INSIDE coax offers the next unit in line a similar phase point (180° out) at it's input, as the output of the preceding unit, while it simultaneously offers an approximate 1/3 wavelength of shield, a much less resonant & less friendly impedance length to any RF which might be looking for another (resonant) path - potentially causing RF to travel between components as Common Mode Current (CMC) on the shield beneath the casing, usually adding distortion to the audio.
73
